- FOR A LESSON: Give one for free. When they become regular, find something your friend can teach you to make a trade.
- FOR A WEDDING: When your performing is considered your wedding gift to the couple, go ahead. For family members, it's best not to play at all so that you can fully share their special moment with them.
- FOR A CONCERT: Avoid playing a concert gratis because of the large required amount of practicing, rehearsing and performing anxiety, unless it's a trade.
- FOR A RECORDING: Play for free on one or two songs. When you play for the whole recording, make sure to get a percentage of the profits.
- FOR AN AUDITION: Do accompany an audition unpaid for your friend. Usually your friend will return the favor by taking you to lunch or coffee.
Picture from http://www.makinghistory.upenn.edu/happenings?page=17