Greenside bunker shots can give a lot of people grief, particularly beginner players. The method though is actually quite simple but it does take trust at the beginning. The biggest mistakes I see in bunkers is that most people tend to set up the same way as they do when they're hitting full shots. Setting up this way however often leads them to hitting too much sand or 'thinning' their shots.
The reason for these inconsistent strikes is that they use to much of the leading edge of the SW. What we want to be using is the bounce of the club. The bounce of the club is the angle that is formed from the back of the club to the front of the club. Now in order to use the bounce effectively in the bunker, it's a very good idea to open the clubface to the target. This means pointing the face around 45 degrees right of the target. When pointing the clubface right also aim your feet around 45 degrees left of the target to ensure the ball still flies straight at your intended target.
When you open the face of a club, you actually add more bounce on the club. This is very handy in bunkers to ensure you don't dig into the sand too much. You want to ensure you strike the sand in a thumping sensation or as Gary Player used to put it 'Like striking a match'.
To ensure you strike the sand correctly using the bounce of the club, the technique we often use is hinging and re-hinging. This means on the backswing you should hinge your wrists and also as you swing through the shot, you should also feel like your hinging back up to the finish. This means you are really utilising the bounce of the club instead of just driving the hands through low like you would do a normal shot.
Try hinging and re-hinging in the bunker next time and if you have any trouble just let me know.