By now I'm sure you know the key to hitting greenside bunker shots is making sand-first contact and skimming your wedge under the ball. In essence, you're purposely hitting it fat. The problem I see a lot of amateurs have is that there's no consistency to where they make contact with the sand--and therefore, there's no repeatability to their shots. How you set up can make a big difference.
With an open stance (aiming left), play the ball off your left heel, but make sure your belt buckle is closer to the target than your sternum. Your left leg should be bent, and you should feel a lot of pressure in that thigh muscle (left photo). When you swing, maintain that feeling of pressure. In other words, keep supporting your body weight with your left side. The mistake is to hang back in an effort to get the ball over the lip (above). This will make it difficult for you to replicate where the club bottoms out. You also run the risk of blading the ball due to a shallow angle of attack.
Sean Foley, a Golf Digest Teaching Professional, works at the Core Golf Junior Academy, outside of Orlando.