The mirror is a great way to practice. Also, you can stand in front of the TV during televised games and practice your whistle and signals.
When I first started, one of the things passed onto me was slow, fast, slow, fast. Slow at the point of the violation or foul, fast as you jog to your reporting spot, slow as you report your foul and fast as you jog to the next position. I actually had a very experienced guy tell me that I was standing too long at the spot with the "stop clock" signal before I indicated the violation. It is easier to go faster than slower.... now that I'm slow enough
