One of the biggest problems is the move from backswing to downswing. Many feel the need to 'push off' too much. This throws their body too far towards the target or they stand up, creating a thinned shot or a completely whiff!

Check out my drill for starting the downswing:

http://members.shaw.ca/gord962/drills/downswing.htm

Good luck and let kme know if you need any further advice!