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1. Exercise.  A 2009 study showed that, on average, every hour that a child was sedentary added 3 additional minutes required for them to fall asleep. [6]  Results for adults, at least with moderate exercise, have yielded excellent results as well. [7]  Some researchers argue that there is no more safe and effective sleep aid than good ol' fashioned exercise.
2. Lose Weight.  Being overweight is associated with a variety of sleeping disorders and often dropping those pounds will help substantially. Weight gain has been strongly associated with Apnea. And one set of researchers wrote that "A weight gain of as little as 10% has predicted a six-fold increase in the odds of developing moderate to severe sleep-disordered breathing" [10] based on the results of this study. [11]
3. Melatonin.  Melatonin has been shown in numerous studies and a variety of populations/cohorts to help with sleep. [1]  I have friends, as perhaps to you, for whom melatonin has been a life saver.  But we should keep in mind that this is a hormone and should be treated as such.  If you are getting very poor quality or limited sleep, this is probably whacking your testosterone, neurotransmitters, raising your stress hormones and so on.  So, in this case, Melatonin may be a godsend in the short term to get your sleep rhythm back on track. Melatonin is now frequently sold in doses less than a mg - 300 micrograms is now quite common - and most experts recommend these lower doses. The reason is that the 1 mg and above dosages that were so common for years delivered was a megadose, i.e. it much exceeded your body's normal levels. I have a friend who was a quasi-insomniac:  Melatonin gave her her life back.  But please read the following cautions as well: CAUTION: There is also a study on quail that shows that Melatonin increases GnIH which in turn will turn off GnRH.  This is potentially bad for us vertebrate males since GnRH effects Leutenizing Hormone and Follicle Stimulating Hormone. However, Melatonin has been extensively studied and, as far as I know, there is no correlation between lower testosterone and melatonin (except in Quail). CAUTION 2: Melatonin is associated with lower seratonin levels.  Of course, if you are low on melatonin from caffeine usage, aging, etc., then a small amount of melatonin may be beneficial.  But this issue has not, as far as I know, been extensively studied. (See NOTE on tasimelteon below.)
4. Magnesium.  Many people are low on Magnesium and this can cause sleep disturbances. If you eat well, low magnesium is probably not an issue for you, but many people swear by taking supplemental Magnesium right before bed. 

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5. Green Tea.  CAUTION:  Studies have shown that Green Tea, primarily through the action of an unusual amino acid called L-Theanine, raises dopamine and seratonin levels all the while inducing alpha (deep wave) activity in the brain.  This produces a calming effect, an almost "meditative" state of mind.  In fact, you will read on the web that some are even suggesting that green tea be taken to induce better quality sleep. And it's true that caffeine does not seem to cause wakefulness in daily caffeine users as a certain "tolerance" seems to be built up.  But I know of no study that shows that green tea or any other product taken right before bed gives one better sleep.  There is a Japanese study that shows that L-Theanine taken before bed gives better quality sleep, but this is a far cry from pumping your blood stream with a 100 mg of caffeince shortly before bed!
6. Stretching.  Stretching the major muscle groups before sleep relaxes the entire body. 
7. Yoga.  Regular yoga practitioners were found in one study were found, not necessarily to fall asleep more easily, but to have improved "sleep architecture". [8] As you age, your slow wave and REM cycles shorten and (male) yoga practitioners showed significantly better results in this area.  In fact, in the case of slow wave sleep, yoga actually completely eliminated the age related decline. (fyi: Vipassana meditation helped with REM cycle maintenance but not slow wave preservation.) Slow wave sleep is the restorative, non-REM part of your sleep and it has been found to be key to blood sugar control. [9] Yoga has also been shown to increase nighttime melatonin levels, which may explain its sleep-inducing powers as well. [19]
8. Glycine. Glycine is an amino acid that the body can manufacture from other amino acids or, alternatively, can be obtained through diet, mostly meats.  Several studies [4] have shown that 2 or 3 grams consumed before bedtime induces deepened sleep.  Glycine appears to be involved, for example, in disconnecting the body from outside stimulus during sleep.
9. Benadryl.  Generic Benadryl is cheap and will knock (most of) you out cold.  If you're sensitive, it may even leave you feeling groggy the next morning.  This is NOT something I recommend every night as any drug, no matter how safe, can be psychologically addictive.  CAUTION:  These older style medications cross the blood-brain barrier and decrease acetylcholine levels.  (Acetylcholine is a key brain neurotransmitter.)  Studies have shown that repeated use can lead to mild cognitive impairment. [18] Again, this is just to get your sleep cycle back on track.
10. Lavender.  Lavender has been shown in numerous studies to be a mild sedative with no side effects.  For example, the prestigious journal Lancet (9/9/1995) published a preliminary study showing that patients smelling Lavender had both longer and deeper sleep. Caution:  Do not apply lavender directly to the skin. You want to vaporize it or put it onto the four corners of the mattress, etc.
11. Socks.  I know it sounds weird but Dutch researchers found that putting socks on at night triggers much more restful sleep.  Roy Raymann, lead researcher of the study, wrote that "increases in the temperature of your feet signal neurons in your brain that cause you to fall asleep".  [2] 
12. Caffeine and Alchohol.  Be careful about these two.  Common sense tells you, and your common sense would be right, that too much caffeince (which is highly variable among individuals) can keep the Z's away.  Alchohol, on the other hand, will put you to sleep but has been shown to lead to poorer quality sleep. 
13. Intercourse.  Intercourse, for reasons unknown, pumps out four times the prolactin [13] as masturbation into your system, and prolactin is the "sleep hormone", i.e. it results in drowsiness and likely improved REM sleep. [12]  Oxytocin is also released which lowers stress and can help the onset of sleep. [14]
14. Doxylamine.  This antihistamine is just behind Benadryl is sleep-inducing power.  You'll find it many OOC nightime, "sleepytime" preparations.  Again, this is anticholinergic, so it's not something you want to do all the time.  However, every once in awhile can give you restful sleep. [15] CAUTION: Only use these short term.  See Caution for Benadryl above.
15. Progressive Muscle Relaxation.  This technique, which involves sequentially contracting and relaxing the major muscle groups, will put you out like a rock. Again, this is a proven strategy to produce a classic relaxation response and is also known for reducing cortisol as well. There are numerous sites with excellent descriptions on how to do this - it is not hard - and it produces solid results with minimal effort and time.
16. Melatonin, Zinc and Magnesium.  A recent study found dramatic improvement in a group of senior insomniacs through supplemental melatonin, zinc and magnesium. [16] The researchers found that participants fell asleep faster, had higher quality of sleep and felt better upon wakening.  Not bad for taking three cheap supplements each day! CAUTION:  The actual daily dosages were 5 mg of melatonin, 225 mg of magnesium and 11.25 mg of zinc. The dosage for melatonin was relatively high and there are some concerns about copper depletion with supplemental zinc.  See my link on Zinc Cautions for more details.
17. Milk Peptides. These are widely used in Europe to improve sleep quality and, according to several studies, can result in dramatically improved cardiovascular and cognitive outcomes.  For example, one study on men using milk peptides found that sleep improvement was so profound that cortisol and blood pressure rises from stress were significantly blunted. [17]  For more information, read this article from LEF.
18. Cortisol Manager.   Anecdotally, this herbal-based product can significantly help with many sleep-related issues. For exammple, I just had a reader who struggled with insomnia write that melatonin helped him fall asleep and Cortisol Manager helped him stay asleep. The primary active ingredient is the ancient Ayurvedic herb called ashwagandha. Ashwagandha alone can produce a relaxing effect, but a couple of other herbal extracts are added along with a little phosphatidylserine (a proven cortisol controlling compound) and L-Theanine. The company provided an in-house study and research for those interested in reading more about it.
19. Meditation.  It is no secret that meditation increases midnight melatonin levels. [19]  So a great, and very natural way to raise melatonin is simply to meditate.  The accompanying study specifically looked at TM (Transcendental Meditation) but very likely works for anything that produces a Relaxation Response. Other studies have also indicated that meditation can increase melatonin levels in the long term. [20] One study of Zen meditation documented its profound changes in brain wave patterns. [21]

NOTE:  There is a new drug, tasimelteon, supposedly with few side effects, that is a melatonin agonist, i.e. a chemical that mimics melatonin. As of this writing in October of 2009, it had successfully passed Phase III trials and seems to have the "green light".  Of course, this begs the question:  why not just take melatonin?  I don't have a good answer for that, except to say that some critics of the supplement industry point out that melatonin supplements are not controlled and some tested samples did not even contain the quantity of melatonin shown on the label.  Personally, I do not see this as much of an issue, especially if you are taking melatonin short term, but my job is to inform.

REFERENCES:

1) Sleep,Sep 1995,18 (7) p598-603;Lancet,Aug 26 1995,346(8974):541-4;Eur J Pharmacol,Mar 6 1995,275(2):213-6;Clin Pharmacol Ther,May 1995,57(5):552-8

2) Am J Physiol Regul Integr Comp Physiol, 2005,288: R1589-R1597

3) Med and Sci in Sports & Exerc,1999, 31:483

4) Neuroscience Let, 2001 Feb 16, 299(1-2):93-6; J Neurosci, 1989 Mar, 9(3):743-51

5) He, Y. Science, Aug. 14, 2009; vol 325: pp 866-870. Hor, H. And Tafti, M. Science, Aug. 14, 2009; vol 325: pp 825-826

6) Arch Dis Child, Sep 2009, 94(9):686-9, Epub 2009 Jul 24, "Falling asleep: the determinants of sleep latency"

7) JAMA, Jan 1 1997, 277(1):32-7, "Moderate-intensity exercise and self-rated quality of sleep in older adults. A randomized controlled trial"

8) Sleep and Biological Rhythms, 4(3):207 - 214, Published Online: 19 Sep 2006, "Evaluation of sleep architecture in practitioners of Sudarshan Kriya yoga and Vipassana meditation"

9) PNAS, January 22, 2008, 105(3):1044-1049, "Slow-wave sleep and the risk of type 2 diabetes in humans"

10) Journal of the International Society of Sports Nutrition, Published: 9 December 2005, Volume 2, "Obesity: Prevalence, Theories, Medical Consequences, Management, and Research Directions"

11) JAMA, 2000 Dec 20, 284(23):3015-21, "Longitudinal study of moderate weight change and sleep-disordered breathing"

12) Journal of Neuroscience, Nov 2 2005, 25(44):10282-10289, "Rapid Eye Movement Sleep Is Reduced in Prolactin-Deficient Mice"

13) Biological Psychology, Mar 2006, 71(3):312-315, "The post-orgasmic prolactin increase following intercourse is greater than following masturbation and suggests greater satiety"

14) http://www.scienceline.org/2006/09/ask-wenner-sex/

15) http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0000703

16) J Am Geriatr Soc, 2011, 59(1):82-90, "The Effect of Melatonin, Magnesium, and Zinc on Primary Insomnia in Long-term Care Facility Residents in Italy: A Double-blind, Placebo-controlled Clinical Trial"

17) Eur J Nutr, 2005 Mar, 44(2):128-32, "Effects of a tryptic hydrolysate from bovine milk alphaS1-casein on hemodynamic responses in healthy human volunteers facing successive mental and physical stress situations"

18) "Medications Found to Cause Long Term Cognitive Impairment of Aging Brain, Study Finds", ScienceDaily (July 13, 2010)

19) Biological Psychology, 1 May 2000, 53(1):69-78, "Acute increases in night-time plasma melatonin levels following a period of meditation"

20) The Journal of Alternative and Complementary Medicine, July 5, 2004, 10(2), "Effects of Hatha Yoga and Omkar Meditation on Cardiorespiratory Performance, Psychologic Profile, and Melatonin Secretion"

21) Psychiatry and Clinical Neurosciences, December 1966, 20(4):315–336, "AN ELECTROENCEPHALOGRAPHIC STUDY ON THE ZEN MEDITATION (ZAZEN)"