I learned something very interesting and simple in the book Yes! 50 Scientifically Proven Ways to be persuasive by Robert Cialdini. They found that if you put a sticky note on a request, the person was much more likely to say yes to what you sent them.
How can you use this idea in the classroom? If you need a parent volunteer for something, just handing out a sticky note to each student and having him or here write a quick note to place on the paper. I have found this increases my volunteers, and does not take much time at all. Even though I work in a school that is 86% free reduced lunch, I still get parent volunteers this way!