No shot in golf comes with more mental baggage than the sand shot. Virtually every amateur, upon climbing into a bunker, starts ticking off a mental checklist of set-up and swing keys -- open stance, open clubface, swing out to in, hit two inches behind the ball, follow through, and so on. As a result, by the time they have to hit the shot, they're so loaded down with mechanical thoughts they can hardly take the club back.
Why do amateurs think so much in the sand? Because they've been told to. They've read the instruction in books and magazines, taken lessons with teaching pros, even listened to their golf buddies (who can't get out of the sand themselves). All this advice leads to either indecision or confusion, both of which defeat the purpose of having a plan in the first place.
Here's a better idea: Keep your approach as simple as possible. Get into a comfortable set-up and simply focus on swinging the clubhead under the ball. Clear your mind and you'll find more success in the sand.
The only true fundamental for greenside bunker shots is getting the center of gravity of the clubface under the center of gravity of the ball. If you do this, the sand you displace will force the ball upward with enough loft for most situations. Don't get me wrong: There are mechanical keys to playing extra-high shots or for generating a lot of backspin, but for a basic, get-on-the-green bunker shot, swinging the clubhead under the ball should be your primary thought.
Set-Up: Simply Square
Swing: Up and Down