Cock Push Ups. For those who don't understand, its the contraction of raising your penis up and down with just the use of your penile muscles.
Get a small-medium towel. Wet it.
Place over erect penis, and perform 300 cock push ups a day, in what ever amount of repetition that is. 100 now 50 later then 150 at night, whichever way, accumulate 300.
Doing so for about 2-3 weeks, will build muscle and stamina.
Received and confirmed this tip from a TV Sex Show on HBO.