For just a few, I use the candle as JB mentioned. For a couple hundred at a time I use a propane torch atop a Coleman gas bottle.
The propane is quite a bit faster.
I set the flame fairly low and roll the case between finger and thumb, while directing the flame onto the shoulder. When the brass gets too hot to hold in my fingers, I drop it and grab the next case.
Some guys drop the hot case into water to ensure the heat does not reach the head of the brass.
