Cock rings are your friends.
I bought a 3 pack of 'Blue' rubber cock rings. They were pretty good, the smallest one had a chip in it and broke away, but not in a painful or embarassing way.
When that one broke, I went back to the super sex shop where I bought the first set, and found a 'Black' set, which were pretty harrrrd rubber; on the small one, was the hardest rubber, which I'm wearing around my balls. I use the medium sized blue one around the back of my shaft and balls, which keeps it nice and tight. I use the medium black one, because it has the most perfect hardness/rubberieness ratio, and that one goes at the base of my penis, but ahead of my balls.
It's quite a fantastic sight, my girlfriend confesses.