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.
Holding your penis, firmly press the skin back into your body by placing both thumbs, nails touching, over the shaft and your remaining fingers underneath for support. Be sure not to cut off circulation to the penis. Hold for 10 seconds then relax and repeat. You want to maintain a continuous pump action. The farther back the penis is held, the better your results will be. Repeat once or twice daily for five minutes. This exercise can also be performed while semi-erect.
Another option for an “emergency” short term size increase is simply having a warm bath, a shower, wrapping your penis with a wet and warm towel or simply pulling it by hand. The blood flow increases and the flaccid size increases, too. The effect only lasts for minutes, but there are situations this may become handy, especially if you were in a cold environment before and your penis size has “shrunk” to even smaller than usual.
You can make your penis look bigger by eating a healthy diet and exercising to reduce belly fat and tone the muscles of your lower abdomen. You can also try surgical methods, but these techniques are expensive and can be risky or ineffective. You might also be able to temporarily enlarge your penis with a vacuum pump, but take care not to overuse it or you could damage the tissues in your penis.
The side effects of lengthening surgeries are numerous and include infections, nerve damage, reduced sensitivity, and difficulty getting an erection. Perhaps most disturbing, scarring can leave you with a penis that's shorter than what you started with. Widening the penis is even more controversial. Side effects can be unsightly -- a lumpy, bumpy, uneven penis.