Setting up with an open stance and cutting across the ball from out to in is the traditional technique used to play greenside bunker shots.
The problem with that method is, it promotes a steep downswing that can make the club dig too deep. Also, the channel created in the sand by this type of swing points left of the target rather than where you're going. I've seen too many shots left in the sand with a swing like that, so I've come up with another way.
You'll have more success if you play the ball farther forward and change your swing path. It's OK to take the club back along your open stance, but swing through straight down your target line (above, left). This shallows your approach into the ball, allowing the club to skim through the sand--not dig. Your goal here is to enter the sand an inch behind the ball and let the club slide under it. With less steepness in your swing, this will become much easier to do.
To avoid three-putting, take into account where you want the ball to stop should you miss your first putt. Ideally, you'll leave yourself with a slightly uphill putt that has very little break. Never leave a downhill approach putt short of the hole--that's the worst mistake. There's nothing wrong with trying to make a long putt, of course, but a good two-putt strategy will result in fewer putts for the round.
LEADBETTER, a Golf Digest Teaching Professional, is based at ChampionsGate, near Orlando. He operates 26 golf academies worldwide.