Update 10/20/2019: A new metastudy done by scietists from Italy and the UK reviewed several non-surgical and surgical approaches to increase penis size and evaluated their risks. The results are fully in line with out own and support the statement that penis surgery and almost all of the devices and pums sold for enlargement present a high risk and very limited results.
The best ways to make the penis bigger naturally are to lose weight in the groin area and for men to do Kegel exercises. “There is almost as much penis inside the body (as an anchor) as there is visibly outside of the body. When a man loses weight, the exterior portion will actually be longer. I believe it is about 1/2"-1" for every 10 lbs lost. It is why often skinny guys seem to be more well-endowed,” says Yelverton. Kegel exercises strengthen the pelvic floor and the majority of men in studies report longer and stronger erections (which men often equate to size).
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.
Lengthening the penis. The most common procedure is to cut the ligament that connects the penis to the pelvic bone. This allows a little more of the shaft -- on average less than an inch -- to become visible outside the body. It's not really lengthening the penis so much as revealing more of what's usually hidden. To prevent the ligament from reattaching, a guy would need weights or stretching devices daily for about six months.