The way I was "taught" to countersink, using a 3-flute tool, was to switch the machine on, then off, and peck the tool in using just the remaining momentum to cut the countersink (this is on a Bridgeport machine). As a technique, it works well, so long as you don't need a deep countersink (it gets a bit tedious, all that flicking the machine on & off).
As others have said; slow slower slowest... decent amount of lube (you don't need to go nuts), and don't put too much downward pressure on - enough to keep it cutting, but don't hang off the handle with all your bodyweight...
I have cut some square countersinks in my time, usually due to using too much speed I feel.