IowaGuy
Hall of Fame
One thing I haven't tried yet is making it look like I'm going to slice approach DTL but hit a DS instead. Federer did this to Berdych and, even though he hit it quite high, he completely froze Berdych because Berdych was expecting something deep.
I've been experimenting with this after Jolly suggested it a while back.
It is surprisingly effective, and IME, your dropper doesn't even need to be as good as one you don't approach on. i.e. even a mediocre DS works well, as long as your motion is similar to your normal slice shot (which Fed does to perfection!).
There is something about the opponent seeing you moving forward (assuming you also approach on deep slices other times in the match) that keeps him on his back heels a little too long to be able to respond well to the dropper.