I was hitting with a minor league player last week and he taught me, in my opinion, the simplest, yet most effective drill I have ever done. Hit off the tee, but focus on keeping your head perfectly still. Don't even look up to see where you hit the ball - just look straight at the hole in the tee. This has helped me tremedously and all the mechanics seem to fall into place when your head is still. Aside from that, I noticed that everything was smoothed out and felt almost effortless.