Keep center of gravity behind the ball, clubface open
For a standard, flat-lie greenside bunker shot the instructor focuses on two things: keeping his center of gravity about one to one and a half inches behind the ball (buttons or zipper as the reference) and not letting it move closer to the ball, and keeping the club face open and pointed to the sky through the entire swing. He says that as long as the center of gravity does not move closer to the ball you won't hit it thin, and as long as you accelerate through you won't hit it fat.
Channel: Micah Morris
Watch official source at 3:02Why this context matters
Standard flat-lie greenside bunker shot, hitting a 56 degree sand wedge.