All you need to do is read page 96 of the little red book by Harvey Pennick. It is labeled the magic move.

This single swing thought (and the 440 cc driver) changed the way I play the game.
He talks about most average golfers and their tendency to slice the ball. Basically what you need to "cure" your over the top swing is when you reach the top of your backswing start forward with your left thigh (instead of your right shoulder) this promotes a smooth weight transfer and avoids the dreaded over the top cast.
Next is the key... as your moving to the left your right elbow will feel like it drops into the "slot" almost touching your right side. If you can get to this position you are gold. From here fire thru the impact zone and finish high. Start walking 300 yards to your ball and get out your wedge
I suggest doing this several times each night to ingrain it into muscle memory. I'd also suggest seeing a pro you trust for some lessons.
Good Luck!