Here is a drill from Michael Breed (PGA Coach of the Year 2012). This drill teaches you to swing on the correct plane. All you need is a coat hanger. I had issues with my swing plane last year and I tried this out. It helped me greatly with my swing mechanics. I did 20-30 practice swings daily with the hanger at home in front of a mirror for about 2 weeks. It felt weird to start, but after a few days started to feel natural.
This is a 2 minute clip from Golf Channel’s, Golf Fix show.