Contrary to popular belief, your penis is not a muscle, nor does it contain a bone. It's better to think of it as a kind of sponge, which, when flaccid, is soft and malleable, but can quickly become engorged when filled with blood. Two large cylindrical chambers, the corpus cavernosum and the corpus spongiosum (see?), fill with blood during arousal, causing your penis to harden and grow in size.
Consider taking L-arginine. This is an amino acid that leads to increased blood flow, helping the penis get larger during an erection. A study conducted at Tel Aviv University showed that some men experienced improvements after taking the supplement for 6 weeks. It's available in natural food stores, and the recommended dose is 1 gram (0.035 oz) three times a day.
Look into taking maca supplements. This powder is known for being an aphrodisiac. It contains the photochemicals macamides and macaenes, which are said to boost energy and help men maintain erections. Because no conclusive scientific studies have been conducted on this substance, it's best to proceed with caution. Be sure to discuss it with your doctor before you start adding this supplement to your daily routine.
The vacuum pump. This is a cylinder that sucks out air. You stick your penis in and the resulting vacuum draws extra blood into it, making it erect and a little bigger. You then clamp off the penis with a tight ring -- like a tourniquet -- to keep the blood from leaking back into your body. What are the drawbacks? The effect only lasts as long as you have the ring on. Using it for more than 20 to 30 minutes can cause tissue damage. This is sometimes used as a treatment for erectile dysfunction, but has not been proven to actually increase the size of the penis.