CockRing

Piece of metal or leather worn around the cock or cock and balls. Restricts blood flow out of the cock but not into it resulting in a firmer, longer lasting boner than you'd have ordinarily.

VenusEnvy, WolfgangLeathers and a number of other GayBusinesses have many different kinds. I've heard the kind that is a rubber shoelace with a clamp to make it different sizes work best.