[grantsrc]

Here are my thoughts, FWIW:
Don't mirror signals.
Don't mirror whistles.
Don't double up flags.
Keep the whistle out of your mouth, IF you are comfortable with it. If not, BE SURE you see the ball prior to blowing the thing.
There is no harm in having a play without a whistle.
On an incomplete pass, there is no harm in blowing a short whistle to signal to everyone else that the play is dead.
There is no harm in having no whistles on a "three yards and a cloud of dust" type running play.
There should never be more than two whistles on any given play. If there are more, too many eyes on the ball.

Just my opinions.