Some sage advice about those times when you do decide to work for free....
http://www.refinery29.com/daily-worth/105#slide
If you're even considering working for free, you're probably in a creative profession — writing, filmmaking, fashion, live performance. It's rare that anyone asks a dental hygienist or a bartender to work for free.
Let's get more specific. I believe in contracts. I believe in quantifying everything. I believe that if someone wants you to work for free, you can take charge of the situation and turn it into cash (or something else of value).
http://www.refinery29.com/daily-worth/105#slide