A cock ring is designed to help maintain an erection longer, but I wouldn't recommend extended use.
The previous post suggests one with a vibrator. That is actually a good idea. We sell many different types of cock rings with a vibrating bullet to help stimulate your partner's clitoris during intercourse. My wife loves the Screaming O and I like it because the ring isn't too tight. The ring is actually very comfortable.
We also sell cock rings that have 2 vibrating bullets, one for her clitoris and the other on the lower side to tickle you. I think I might try one of those next.