Short of riding a manual/wheelie up the hill on a nearly deflated tire, there is no magical cure. Deep sand kills traction and momentum as Colin pointed out. What I try to do is sit up fairly straight on the saddle, keep the pedal stroke consistent and light pressure on the handlebar. My hands are merely suggesting the direction of travel to the front wheel. trying to keep balance and don't freak when the tire slips, stay balanced and keep a steady pedal stroke going.