Practicing the free stroke has helped me tremendously to get my left hand working. I can honestly say that 6 months ago, I couldn't do it with my left hand. I kept dropping the stick, had trouble letting it rebound. I realize now of course, my left hand technique on the kit must have been a disaster!
By now I can play clean 16th note singles at about 90 bpm with my left hand for a few minutes, and I can pull off 32nd notes with both hands at 100bpm for 1-2 minutes. Of course my left hand is still retarded compared to my right but it's getting easier :)