I'm by no means an accomplished freestyler, but i had the same problem a while back. The advice that corrected it for me was to lean more forwards in my boots as i took off, and to make sure my leading hand (left if u spin left) stayed low. my problem was that i flailed my arm upwards and it threw my balance. If this is completly wrong then someone correct me, otherwise i hope it helps!
Skiing: the art of catching cold and going broke while rapidly heading nowhere at great personal risk.
manwiththemetalleg - "Note: about 0.01% of this site ski on dryslope. 80% don't even know what one is."