The proven medical method to enlarge a penis is basically by stretching the penis longer. This can be acheived by using penis extenders or performing certain exercises to stretch the penis.
There are many skeptics, but lets use a bit of logic. Your penis is like any other area of your body - if it is not used correctly, then it will slowly become weaker (your muscles work this way and your brain does too). Only by performing the correct exercises can you ensure your penis is as large as it could be.
By performing certain exercises such as jelqing and schlong many men have successfully increased the size of their penis and their testimonials show their success after performing the exercises. These exercises have to be performed following a strict program in order to gain maximum size for your penis. Most penis enlargement programs have very clear pictures and videos on how these exercises are to be performed.