Belt slipping off.

I've got a 97 Dakota and the belt (serpentine??, runs the AC, power steering, etc...) keeps slipping off. When it doesn't slip off it hangs over the edge of the..pulleys?...and eventually breaks at the most inopportune time. I've worked on it for awhile and even when it's set perfectly it only takes a few seconds of the engine running for it to start slipping off. Does anyone know whats wrong?

Make sure the tensioner is not froze up. And while the belt is off check all the pulley for play and make sure they spin easily.