I've always had a very poor non-visual memory. I think there are some logical explanations for this, including a tie to my low testosterone, which I discuss below. And, of course, it doesn't help to be 55 either. The good news is that I feel I am doing very well overall - I am able to learn new things in both of my careers quite easily due to hormone and lifestyle changes. But some aspects of my memory could definitely use some "bolstering," and so below I cover 10 Ways to Improve Your Memory According to the Latest Research:
1. Sleep. None of us want to hear this, but by far the most potent and research-backed memory enhancer is sleep. Study after study has shown this. There is even a study that shows that going to sleep after studying improves memory recall! I doubt that anyone is shocked by these statements, so I will just cite one glowing research review to hopefully get you excited about retiring a bit earlier tonight:
"Our study provides further evidence for the notion that memory consolidation in the declarative memory system benefits from sleep. It extends previous studies in that it shows that consolidation is enhanced when the interval of intervening wakefulness between learning and sleep is short. It also shows that this beneficial effect of sleep is stable over 48 h. These findings are independent of time of day and not due to acute fatigue. Together with previous studies, these data encourage the idea that, for optimal retention, phases of intensive learning, like school, should be followed closely by intervals of sleep." 
Notice that the authors basically describe sleep like a supplement: take it if you want to improve your memory for work, school and life in general. And, yes, research shows that sleep deprivation hammers memory as expected. 
2. Weight Lifting / Strength Training. A recent study showed the power of just one (relatively short) strength training workout:
"A single, brief session of resistance exercise done immediately after a visual learning task enhances episodic memory by about 10%, new research shows. Lisa Weinberg, a psychology graduate student at the Georgia Institute of Technology, in Atlanta, and colleagues found that a resistance workout lasting as little as 20 minutes improved recall of a series of photos shown to participants 48 hours earlier." 
Since I love lifitng weights, that's great news if you ask me.
3. Exercise. Right behind sleep as a memory booster is exercise. Of course, it will help you tremendously in the bedroom as well, boosting your nitric oxide and physical endurance as you do mighty deeds of valor. But, most importantly, you will be able to remember what happened.
Many studies have shown the powerful pro-memory effects of exercise (and many other forms of cognition and learning) from exercise, but check out this summary:
"The hippocampus shrinks in late adulthood, leading to impaired memory and increased risk for dementia. Hippocampal and medial temporal lobe volumes are larger in higher-fit adults, and physical activity training increases hippocampal perfusion, but the extent to which aerobic exercise training can modify hippocampal volume in late adulthood remains unknown. Here we show, in a randomized controlled trial with 120 older adults, that aerobic exercise training increases the size of the anterior hippocampus, leading to improvements in spatial memory. Exercise training increased hippocampal volume by 2%, effectively reversing age-related loss in volume by 1 to 2 y. We also demonstrate that increased hippocampal volume is associated with greater serum levels of BDNF, a mediator of neurogenesis in the dentate gyrus. Hippocampal volume declined in the control group, but higher preintervention fitness partially attenuated the decline, suggesting that fitness protects against volume loss. Caudate nucleus and thalamus volumes were unaffected by the intervention. These theoretically important findings indicate that aerobic exercise training is effective at reversing hippocampal volume loss in late adulthood, which is accompanied by improved memory function." 
Yes, you read that right: exercise actually grew the hippocampuses of senior age adults and improved memory! That should be front page news, of course, but its not a pharmaceutical and so the medical community yawned instead.
4. Meditation. There is so much research showing the positive effects of meditation on memory that it is difficult to summarize it quickly, but I'll do my best below:
a) Meditation is associated with a larger hippocampus, the part of the brain that is associated with memory processing (and memory loss in many seniors). Researchers stated that "We detected significantly larger gray matter volumes in meditators in the right orbito-frontal cortex (as well as in the right thalamus and left inferior temporal gyrus when co-varying for age and/or lowering applied statistical thresholds). In addition, meditators showed significantly larger volumes of the right hippocampus."  A more recent study actually looked in even more detail and found even more positive results:
"Left and right hippocampal volumes were larger in meditators than in controls, significantly so for the left hippocampus. The presence and direction of this global effect was confirmed locally by mapping the exact spatial locations of the group differences. Altogether, radial distances were larger in meditators compared to controls, with up to 15% difference. These local effects were observed in several hippocampal regions in the left and right hemisphere though achieved significance primarily in the left hippocampal head. Larger hippocampal dimensions in long-term meditators may constitute part of the underlying neurological substrate for cognitive skills, mental capacities, and/or personal traits associated with the practice of meditation." 
b) Okay, so meditators have a more dense memory-related brain structure than non-meditators. So then does meditation actually improve memory? The answer is, yes, at least in those that are experiencing some memory issues. As an example, there is a recent study that shows that in those experiencing memory loss, the brain of those put on a meditation program were actually larger, i.e. the meditation slowed the hippocampal shrinkage in that area.  More directly, another recent study in the Journal of Alzheimer's Disease found that, not only was brain blood flow improved in those who meditated after experiencing memory loss, but that "scores on neuropsychological tests of verbal fluency, Trails B, and logical memory showed improvements after training." 
5. Testosterone in Hypogonadal Males: Several studies have shown that low T is correlated to poor memory. And it's no wonder considering that low testosterone increases inflammation, heart disease and is hard on the brain according to many studies. The question is if giving testosterone to low T guys could improve memory. One study showed that it did just that, improving working and verbal memory. 
CAUTION: Men on TRT should be cautious about allowing their LH to drop to near zero, which is very common after being on TRT for awhile. The hippocampus is packed with LH (luteinizing hormone) receptors. Since I was likely low T my entire adult life (and also likely secondary hypogonadal), perhaps this explains some of the issues I've had with memory?
6. Omega-3's / Fish Oil. Studies shows correlation between omega-3 levels and memory and cognitive abilities in adults. The question was whether or not giving someone fish oil could actually improve memory. Of course, improving memory in young adults is no small task. However, scientists gave high-DHA fish oil to adults with mild cognitive impairment, "early Alzheimers" and noticed marked improvments:
"The fish oil group showed significant improvement in short-term and working memory, immediate verbal memory and delayed recall capability. The 12-month change in memory was significantly better in the fish oil group. Fish oil consumption was well tolerated, and the side effects were minimal and self-limiting. This study suggested the potential role of fish oil to improve memory function in MCI subjects. Studies with larger sample sizes, longer intervention periods, different fish oil dosages and genetic determinations should be investigated before definite recommendations can be made." 
All I can say is that I am glad I consume my daily BPA-free, low mercury sardines!
7. Sage and Sage Extract. This ancient spice now has two studies under its belt in both (remarkably) young and older subjects.  The study that found improvement in younger adults stated that "The 50 l dose of Salvia essential oil significantly improved immediate word recall in both studies. These results represent the first systematic evidence that Salvia is capable of acute modulation of cognition in healthy young adults."  Again, anything that improves memory in young adults, who have incredible memories generally to start with, is worthy of taking note.
8. Choline? Hmmm. Savvy readers, when any discussion of memory comes up, will think of acetylcholine, the "memory neurotransmitter." Boosting acetylcholine and memory should be as easy as taking the this common precursor, right? There are phophatidylcholine supplements available or one can take lecithin. As easy and common sense as this method would appear to be, there are several major issues with this approach:
a) Poor conversion rate to phosphatidylcholine.
b) Standard forms of increasing plasma choline may not do so for the brain acetylcholine in seniors. 
c) Oral forms of choline may increase plasma levels of TMAO, which a couple of recent animals studies have hypothisized are a root cause of heart disease. The reason is that it alters gut bacteria. Calcififying the brain with plaque may be a root cause of Alzheimer's, so this would not be a good thin.
d) Although poorly understood, phosphatidylcholine in particular is found concentrated in many cancer tissues. Is this causative or coincidental?
There are other ways to try to increase brain acetylcholine that one can consider, but these are new and certainly have no long term safety studies:
NOTE: Anything that boost plasma acetylcholine levels will likely improve erections a little. Acetylcholine is a vasodilator, although less potent than nitric oxide.
CAUTION: Whatever you do, do not take any anti-choinergic. Benadryl, and the old school antihistamines, are common examples of these acetylcholine-lowering drugs. (Benadryl is a common example that some men take to improve sleep but do not realize they may be hammering their brains.) Why the concern? They have been shown in seniors to lead to cognitivie and memory damage.
a) CDP (Cytidine Diphosphate) Choline. This type of choline is natural in the sense that it is an intermediary in your body's metabolic process. And researchers observed the same of lack of uptake in brain choline observed above after giving patients the CDP form.  However, the authors believe the intracellular cytidine was combined with the choline that entered the brain to actually increase brain levels. This is substantiated by the fact that researchers have achived memory improvements / preservation in a few animal and human studies with CDP choline.
The issue in my mind is dosing and a lack of long term safety studies. However, CDP Choline has been given to Parkinson's patients with a good safety profile from what I have read but discuss with your physician of course. Check out this statement from one set of researchers: "CDP-choline has also been shown to be effective as co-therapy for Parkinson's disease. No serious side effects have been found in any of the groups of patients treated with CDP-choline, which demonstrates the safety of the treatment."  That doesn't guarantee long term safety of course, but it's a good start and a good sign.
b) Piracetam. Piracetam is arguably the "Mother of All Nootropics." Rather than increasing brain acetylcholine levels, it just directly stimulates that receptor. The problem in my opinion is that it seems to have a fair amount of anecdotal side effects.
9 and 10. Yoga and Progressive Muscle Relaxation. Several mind-body practices, including yoga and Progressive Muscle Relaxation, now have a study or two under their belt to show that they improve memory. 
MAYBE: 7-Keto DHEA, DHEA and Pregnenalone. These have been understudied and poorly studied. Pregnenoone and and 7-Keto DHEA have solid animal studies showing memory improvements. And DHEA has a study given to young men that showed the same. However, the problem with this study is that young me don't really need DHEA and the dose they gave was very large. I think it is reasonable to assume that those over 50, with confirmed low levels, that are given low dose DHEA and pregnenalone will likely experience some improvements in memory. Check out these links for more information: Summary Page on DHEA and Summary Page on Pregnenolone
1) NeuroImage, 15 April 2009, 54(3):672 678, "The underlying anatomical correlates of long-term meditation: Larger hippocampal and frontal volumes of gray matter"
2) Human Brain Mapping, Dec 2013, 34(12):3369 3375, "Global and regional alterations of hippocampal anatomy in long-term meditation practitioners"
3) Neuroscience Letters, Nov 2013, 556(27):15 19, "Meditation's impact on default mode network and hippocampus in mild cognitive impairment: A pilot study"
4) Journal of Alzheimer's Disease, 12 Jan 2010, 20(2):517-526, "Meditation Effects on Cognitive Function and Cerebral Blood Flow In Subjects with Memory Loss: A Preliminary Study"
5) JAMA, 1995 Sep 20, 274(11):902-7, "Decreased brain choline uptake in older adults. An in vivo proton magnetic resonance spectroscopy study"
6) Psychopharmacology (Berl), 1996 Sep, 127(2):88-94, "Differential effect of CDP-choline on brain cytosolic choline levels in younger and older subjects as measured by proton magnetic resonance spectroscopy"
7) Methods and Findings in Experimental and Clinical Pharmacology [1995, 17 Suppl B:1-54],"CDP-choline: pharmacological and clinical review"
8) Psychopharmacology, Feb 2013, 225(3):605-612, "Docosahexaenoic acid-concentrated fish oil supplementation in subjects with mild cognitive impairment (MCI): a 12-month randomised, double-blind, placebo-controlled trial"
9) Psychopharmacology, May 2008, 198(1):127-139, "An extract of Salvia (sage) with anticholinesterase properties improves memory and attention in healthy older volunteers"
10) Pharmacology Biochemistry and Behavior, Jun 2003, 75(3):669 674, "Salvia lavandulaefolia (Spanish Sage) enhances memory in healthy young volunteers"
11) Learn. Mem, 2006, 13:259-262, "Sleep after learning aids memory recall"
12) PNAS, 2011, 108:3017 3022, "Exercise training increases size of hippocampus and improves memory"
13) http://www.medscape.com/viewarticle/833066, "Strength Training Boosts Memory in a Single Session," by Pam Harrison, October 10, 2014
14) BioPsychoSocial Medicine, Published: 13 August 2009, "Effect of two yoga-based relaxation techniques on memory scores and state anxiety"
15) Experimental Aging Research, 1982, 8*4), "Relaxation training and memory improvement in elderly normals: Correlation of anxiety ratings and recall improvement"
16) Neurology, 2001, 57(1):80-88, "Testosterone supplementation improves spatial and verbal memory in healthy older men"