I use to whistle or go psst (how can you describe this sound? lol), but found that doing so would spook the deer at times. I had my 10-point from 3-4 years ago run an extra 5 yards after whistling at him, however, he still offered me a quartering shot which I took.

Since then, I have resorted to a sound that resembles a "baaaaa" sound. I don't do it very loudly, just enough to make them stop. It's worked pretty good so far. I'm sure you will get NUMEROUS answers to this question tho.