Drop shot approach shot. You make it look like you are gong to slice it deep and open up the racket face at the last second. Also, When you have an easy forehand at the service line, set up like you are going to hit a huge forehand, and at the last second open, up the racket face and drop it. It usually doesn't even have to be a good dropshot because they are so far behind the line because they think you are going to hit a deep forehand.
The exact opposite of this works too. If you're drop-shotting quite a lot during the match the opponent will start to spot the open racquet face, chip it deep and you're almost guaranteed a weak response as they half volley it from the baseline for an easy put-away