What do I use? These days, I use artists' quality spray fixative, but at a time when money was tighter and I was doing a lot more in charcoal, I used a hairspray. It was always one of the cheapest, and I used to try to find the one that ponged the least.
About.com's Drawing Guide, Helen South, says she prefers "to use a reputable brand of fixative" but even then doesn't usually "spray the top layer of pastel and pencil due to color alteration." If you've never used spray fixative, read Helen's explanation on how to apply spray fixative.
Basic Techniques for Pastels
Pastel Problem Solver