Although I personally have not used a decoy (yet), a couple of my buddies have tried them. One of the guys mentioned about putting a tail on the decoy and having some method of making it move or flip.
I've heard of people putting an actual deer's tail (from a harvested deer) on the decoy and using a small piece of string or fishing line to twitch the tail at the right moment. Some regions may not allow using actual animal parts though, so check on your local game laws before trying this.
You could also use a piece or kleenex (or similar tissue) that might move in a slight breeze. I think they also sell decoys that have mechanically driven tails that can either operate off remote control or timed.
In any event, my one friend that tried the tail twitching, said he had good results on bucks that hung up or ignored the decoy at first.