To back up something Liza said - if you are going out to race you should always warm up, either on the course or on a trainer - you want to get yourself going to the point that you feel your legs burn at least a couple of times and try to finish your warm up 1/2 hour or so before the race starts. I find that I usually need to warm up in stages. I spin for a while just to get the body moving, then I take it up to my zone 3 for a while, back down, spin for a while, take it up to my zone 4, etc. It takes some time before I feel comfortable getting my hr up high and my legs moving, so its essential to have a good warm up in before I go out and try to race.
Cyclocross is a short time/high intensity kind of racing that makes a good warm up even more essential.