On a one handed backhand, try to hold your back (off) shoulder back as much as you can. Try pointing at the back fence as you strike the ball. This gives you a base with which to explode thorough the ball from. If you allow you back shoulder to follow your front shoulder, you'll be sacrificing quite a bit of power, and consistency.

KEEP YOUR BACK SHOULDER BACK.

Muster backhand animation

Notice, in this animation
of Thomas Muster's
topspin backhand, his
right arm (since he's a
lefty) stays back through
the entire shot.

The only part of Muster's backhand I would not recommend for beginning and intermediate players is the follow-thru with the racket behind his head. This is due to the heavy top spin he tries to generate. The follow-thru on a good consistent back hand would be through the ball and in front of you.

I still remember when this little tidbit finally seeped into my thick skull. My backhand improved immeasurably.