Help spot a potential girl. A good wingman knows when it's time to help his friends. If you got the girl last time, tonight it's your buddy's turn.
Entertain the friends. Girls almost always go out in groups. A good wingman will keep her friends busy, and give the main man time to talk one-on-one with the girl.
Be a blocker. A good wingman should be ready to distract other guys that could be approaching. Remember, you have to give the main man time alone with the girl.
Provide backup to the main man's stories. Be ready to confirm details if the girl asks you.
Do not steal the spotlight. A good wingman lets the main man work his magic. Part of that means a good wingman must turn his charisma way down. Do not be funny or charming.
Do whatever you can to let your main man look good. This includes telling the main man if it looks like he's making a fool of himself or if he's got no chance with the girl he chose.
Bring up the main man's confidence. Rejection comes to us all, and it will certainly come to the main man every once in a while. A good wingman inflates his buddy's ego enough to keep him in the game, even after several rejections in one night.