There are several inherent problems with the onehander that all you who hit it must begin to work on or understand, otherwise you will continue to be great one day and messing up the next, or great one set and terrible the next.
Revolutionary Tennis puts it in the best way possible. I provide links below to three of the most important things to learn from this site. I think Mark's findings are right on. I don't go as far as he does by downplaying the "establishment" afterall I belong to the "establishment" but I think that a lot of his stuff is good and can really help improve your onehanded backhand. His information on balance, movement, vision and the use of the legs I think is very good.
Vision: Keep both eyes on the ball
Moving to the ball: Move the right way to return properly
Balance and Footwork:You must improve the use, strength and coordination of your non-dominant leg