Argh, we should probably start a whole different thread on this one. I have trouble with it too.
It's so ironic, the shot the pros make look so easy for the most part is arguably the most difficult shot to master for an amateur. I think most decent amateurs have more than enough power to hit an effective overhead, it's just in the timing that it all goes wrong. The ball comes down at completely different speeds and angles depending on so many factors, I see so many 'easy' smashes shanked into the back fence.
I have much better technique for my backhand smash than forehand, so for the conventional overhead I've taken to adding more slice to control it and letting the ball bounce in some situations to make life easier.