Place a ball in the sand and then draw a line 4 inches behind it. Now make your bunker swing and enter the sand where the line is. The ball should pop out on to the green. I still use this drill because it's very effective way to ingrain the concept of the bunker swing: you hit the sand, not the ball. So get your mind off the ball and focus on hitting that line in the sand. When you get more advanced, do this drill with a 9-iron or 8-iron. Using a less lofted club will teach you the swing you need to escape the sand and put it close almost every time.