As a wing, never, never, never call intentional grounding!
Edit: In all seriousness, I can give you 2 tips: keep the whistle out of your mouth and see the ball. My mechanic as a wing is to immediately spit the whistle out of my mouth at the snap. And don't even reach for the whistle unless you see the ball dead (i.e. in player possession if downed or incomplete). If you think you see it dead, stop and think, then reach for your whistle.