When I'm asked to sit on a board I ask myself:

1. Will this entertain me and will I have fun?
2. Can I contribute something of value?
3. Will these people listen to my advice if they aren't paying for it?
4. Will I learn something or become connected with other board members who matter to me?

All of these factors play a big part for me. They're all valuable components. So if too many don't add up in my favor I pass on the board.