What to do if trapped on top VFR?
You should have an alternate planned that is forecast VFR, with appropriate fuel reserves in place, before you left.  But if you didn't, you'd call an ATC unit and tell them your predicament.  Maybe they can send you to VFR conditions either reported by PIREP or by calling around to other tower units in range.

Otherwise, you're going to have to do an instrument let down.  If you're in contact with an ATC unit, let them provide you with a location to do let down that gives obstacle clearance, and hope your instrument skills are up to the task. In Canada VFROTT is not allowed unless you've had at least the commercial pilot level of instrument training (there is a special rating if you don't). Up here it would be considered crazy for a VFR pilot with no instrument training to do VFROTT in the first place, even if it was legal.

Say you didn't have contact with ATC for some reason.  Well than I'd locate a decent size body of water with my moving map GPS and let down over that, and stop descending if I see water.

That's another planning factor.  You want the cloud you are going to be flying over to have a decent VFR ceiling under it, so you only have to let down through a layer into nice VFR below.