junk792: I disagree, oral sex is very effective for penile enlargement... for a few minutes ;D
More seriously, say two identical twins were separated at some point before their puberty, and for one reason or another one grew up sexually repressed - i.e. no sex/masturbation until past the teenage years. And the other twin had plenty of sexual activity - would using the penis (a lot) tend to increase it's size? I don't imagine anyone has made any studies like that... Any anectodal evidence?