Many players leave shots short, so why do you want backspin, and be shorter still? I understand wanting to play a shot with back spin -- however this shot should be played at the right time and place -- making sure you hit a good shot... everytime.
With wedge shots, I feel many players make a mistake in playing the ball too far back in their stance, taking loft off the club. This does not allow you to utilize the correct loft of the club and therefore affects the height of the shot. So don't play the ball too far back in your stance when attempting to put backspin on the ball.
With the proper loft in place, allow the club to slide under the ball -- this will create natural loft -- let the club do the work. You can assist this process by some good leg action -- keeping the club moving through the ball. One special note on short shots of 40 yards and closer, don't expect much spin, as you are not able to generate much club speed for the spin you need..
Get your loft, land it soft and enjoy the result.