good advice in here.
in my experience its pretty hard to ski powder properly if your tips wont rise. ie. very difficult on centre mounted skis.
if its deep really try to keep your feet together as though you were skiing moguls. that makes your base pretty much as wide as a snowboard and you'll float. spring up to turn, pole plant and you'll be fine. oh, and i find a medium speed is best. not too slow but not so fast you cant stop in control.