I'm of the opinnion that most people are more creative than they think they are. My only real evidence of this is the fact that all of us used to be 5 years old and I've never seen a non-creative 5 year old. Somewhere along the line, many of us 'grow up' and outgrow our creativity because it isn't as well accepted in a grown up world. Over time, we forget how to be creative and begin to believe the myth that we aren't.
I stumbled across this website the other day and thought it had some pretty good reminders about creativity and some exercises to get the juices flowing.