Maintain an upright body position over the leading foot. If your bum isn't over the board, you are probably pushing it away from you with each step that you take. When I skate, I find myself drifting to the side, and the board spins around a lot as I skate around. Also, my edges tend to catch which often spins me around and causes me to fall. If you let your board go flat, there is little friction between the snow and the base of your board.
Gravity takes over.
If your ride is spinning around backward, your weight is over the tail. The way to counter this is to shift your weight tipward and use your edges. The idea is to stabilize yourself with the metal edge and still have as much P-Tex contacting the snow as possible for maximum speed on the flats. If you are drifting or being bounced about from edge to edge, pick up and edge, get on it, and stay on it using as little edge angle as possible. Answer Leaning may be simpler, but learning to use your legs to manipulate the board is a much more effective method.