Have you tried jumping without stirrups? That should be a good exercise.
Or.. An exercise I haven't tried but I heard of someone doing in a clinic (I think with Joe Fargis?)... pull your stirrups off your saddle (keep the irons on the leathers) buckle the leathers together so they make a big loop (with irons hanging off the end). Place that loop over the seat of your saddle and ride with your stirrups like that. So you aren't exactly stirrupless, but you certainly can't lean on one stirrup! Then tell me how it goes 'cause I'm dying to hear someone else try it! :)
Also make sure you are very centered in your saddle on the flat. And have others watch you. Chances are if you lean over fences somewehre along the way you're leaning on the flat.