I agree with most of what was said above. Fit is very important. You should have a tiny bit of bend in your knee when your leg is fully extended, so that you don't need to rock on the saddle.

I've done drills with a friend to help her raise her cadence. What we did was find an isolated loop with a clear straightaway (maybe 1/4 mile); we'd drop into a low gear and sprint like crazy on that straightaway, and then recover for the rest of the loop. Do that about ten times, every day for a couple of weeks. You'll notice a difference.