Do you have normal semen volume? Researchers say it is usually between 2 and 6 ml.  with an average of 3 ml. (A teaspoon is 5 ml). Keep in mind that you lose a little with aging as well, about .03 ml/year according to one study.  Semen volume has a lot to do with simple genetics and anatomy. Very roughly half of the semen comes from the prostate, where it fills up the ejactulatory tubes upon stimulation. The tubes are kind of like the missile tubes in a sub ready for launch. The PC muscles contract during orgasm and deliver the goods along with another batch of fluid from the seminal vesicles in the testes for the final explosive, potentially life-changing mixture.
However, some men notice their semen volume has gone down and become concerned. Of course, this can be from ejaculating too much and also from aging. It can also be from retrograde ejaculation as I dicuss in my link on Causes of Low Semen Volume. If it is indeed retrograde ejaculation, it needs to be handled a little differently and so I recommned that you read the above link.
However, in most other cases a man should be able to increase his semen volume a little and I discuss these below. (It is always a good idea to talk to your doctor about it, especially if you are concerned about fertility.)
1. Increased Libido. There's a belief out there that libido is boosted greatly by the tank "filling up and needing emptying". I don't know of any scientific evidence for that, but many men are convinced that this is the case.
2. HRT (Hormone Replacement Therapy). I find that some men on testosterone therapy want to increase their semen volume, because they feel they have lost so much. And they may have. However, the reason is usually not so much the HRT as simply being hypgonadal, which is associated with decreased semen volume in men who have secondary hypogonadism. This condition works from dysfunction of the hypothalmus or pituitary and is characterized by low or low-normal LH (leutinizing hormone).
3. Inadequacy. For many men semen volume is a symbol of their identity as a male.
Regardless of the reason, I have some natural ways to increase semen volume that have a study or two behind them. These are listed below:
HO HUM: Of course, there is always the ultimate boring way to increase semen volume: abstinence. Well, that's a lot of fun, eh? It is true that if you wait a few days between ejaculations, your semen volume will increase accordingly. However, I am not even going to include this as an option, because, unless you are going to join a monastery or switch careers to Arctic Explorer, this is probably not a good option for you.
1. Maca. The herb from South American is synonymous with sexual passion and energy. And it is no wonder: it was used for ages by the ancestors of the Incas for increasing libido. Interestingly enough, this herb, which often served as food for the native peoples, also increased semen volume as well. I cover this in my link on The Superpowers of Maca.
2. Zinc. One study put young men on a controlled diet and varying levels of zinc supplementation . Those with the least zinc had about a third less ejaculate than those with highest.  For more information, see my link on Zinc and ZMA.
NOTE #1: If you are on HRT, there are no guarantees that any of these natural methods will increase your semen volume. The studies are typically not done on males undergoing testosterone therapy obviously.
3. Tribulis. Tribulis Terristris is what I call one of the Super Sexual Herbs. Admittedly, it has a spotty record in raising testosterone levels in men. However, this herb is known, among other things, for mimicking leutinizing hormone and, as such, it should not be surprising that it boosts semen volume in a couple of animal studies.  Does it in humans? Well, you may have to just try it to see if it works for you. (Always check with your doctor first of course.)
4. Vitamin C and Vitamin E? One study showed that daily megadoses of Vitamins C and E on infertile men did not significantly improve semen volume.  However, the jury may still be out as a study on rabbit just a few years later showed an increase in ejaculate volume with the same two vitamins. So it may be dosage levels, times per day, etc.  Also, I always recommend mixed tocopherols if you take Vitamin E as taking just the traditional alpha tocopherol form will lower levels of the tocopherols (gamma and delta, which is not a good idea as these serve very different functions generally).
NEWS FLASH: There is some decent evidence that sperm acts as an antidepressant when absorbed through the vagina post-intercourse and can even lower her blood pressure when taken orally. 
5. HCG. Some of my might bristle a little that I would call this "natural". Well, HCG is natural in the sense that it is a placental hormone that the body is used to at some stage in its development. It's remarkably similar, as well, to leutinizing hormone and so the body responds extermely well to it. Its use is on the rise with men on HRT, who have some shrinkage of the testes. HCG allows men to stay on testosterone therapy with no loss in volume. (HCG can really ramp up estradiol production, so regular monitoring is required. See my link on Testosterone and HCG for more information.)
And, as expected, HCG will ramp up semen volume in men where in many men. In one interesting case study, one man was found to be low testosterone from a pituitary disorder. HCG restored his semen volume (and testosterone).  Similar strategies have been used in young men with secondary hypogonadims as well. 
6. Prolactin. High prolactin generally means lower testosterone and lower libido for us guys. And, interestinly enough, one study shows that it can mean decreased semen volume as well.  If you feel you're not squeezing out the juice like you used to, you might want to have your prolactin pulled as well, For more information, see my link on Prolactin and Prolactinomas.
7. Retrograde Ejaculation. This condition occurs when some of the semen actually gets pushed into the bladder upon ejaculation. How would something this occur? Actually, it is surprisingly common, because there is a valve (sphincter) that normally keeps this from occurring. A number of things (surgery, drugs, local nerve damage, etc.) can lead to a state where this valve becomes overly relaxed and semen is allowed to flow down the wrong path. Men with this condition may notice that they literally have no ejaculate upon orgasming. If so, talk to your urologist.
8. Diabetes. Retrograde ejaculation (#5) can occur due to nerve dysfunction or damage, because proper signaling never reaches the sphincter valve. Of course, neuropathy is one of the hallmark symptoms of diabetes and so it is no wonder that this condition is so common. In fact, one study found that diabetic men actually had increased sperm output and sperm concentration but decreased semen volume and they speculated that this was due retrograde ejaculation. 
9. Smoking. We all know smoking is bad for you, but if you've never quit given them up, keep in mind that they will likely reduce your semen volume as well. One study looked at fertile men and grouped them into non, mild, medium and heavy smokers. What is interesting is that hormone levels (testosterone, LH and FSH) as well sperm motility (mobility) and density did not differ significantly among the groups. The one characteristic that did differ was semen volume, which decreased with increasing levels of smoking. 
10. Cadmium. High levels of cadmium were correlated with lower semen volume in one study. 
1) Am J Clin Nutr July 1992 vol. 56 no. 1 148-157, "Effects of dietary zinc depletion on seminal volume and zinc loss, serum testosterone concentrations, and sperm morphology in young men"
2) Biotechnology in Animal Husbandry, 2008, 24(3-4):139-146, "Effect of Tribulus terrestris extract on semen quality and serum total cholesterol content in White Plymouth Rock-mini cocks"
3) Hum. Reprod, 1999, 14(4):1028-1033, "Antioxidant treatment of patients with asthenozoospermia or moderate oligoasthenozoospermia with high-dose vitamin C and vitamin E: a randomized, placebo-controlled, double-blind study"
4) Animal Reproduction Science, Mar 2003, 76(1-2):99-111, "Effect of ascorbic acid and Vitamin E supplementation on semen quality and biochemical parameters of male rabbits"
5) Urology, Oct 2000, 56(4):669, "Acquired hypogonadotropic hypogonadism presenting as decreased seminal volume"
6) Fertility and Sterility [1991, 56(2):319-324], "Subcutaneous gonadotropin therapy in male patients with hypogonadotropic hypogonadism."
7) Fertility and Sterility, 1979, 32(5):556-561, "Male hyperprolactinemia:effects on fertility"
8) Systems Biology in Reproductive Med, 1993, 30(1):47-54, "Semen Analysis in Insulin-Dependent/Non-Insulin-Dependent Diabetic Men with/without Neuropathy"
9) BJU International, Feb 2006, 97(2):324 326, "Cigarette smoking is related to a decrease in semen volume in a population of fertile men"
10) Systems Biology in Reproductive Medicine, 1992, 29(2):177-183, "Blood Concentrations of Lead, Cadmium, Mercury, Zinc, and Copper and Human Semen Parameters"
11) "Speman in Male Sterility", [Probe (1970): (IX), 4, 159], Vaze, V.H., M.S., F.C.P.S., Ambai Hospital, Kolhapur, India.
12) Hum. Reprod, 2003, 18(2):447-454, "The association of age and semen quality in healthy men"
13) http://www.nbcnews.com/id/5263250#.UXyUd7U4tPM, "Not just good, but good for you", By Brian Alexander (msnbc.com contributor)