It looks like it's a common enough problem that bicycle seat companies are using it (or lack of it) as a selling point for their seats...

Reduce Perineal Numbness With A Derri Air - Bicycle Seat.

Here is an article that gives you some basic advice about preventing penile numbness.

Health24 - News, Your equipment

and here is an article that will put the FEAR OF GOD into your penis (impotency my friends)

Are You Exposing Yourself To Unseen Dangers By Riding A Bicycle? Derri-air Can Help