Since mountain biking is an endurance sport, success relies on good pacing, yet it's an area where many new racers make mistakes. Pacing is a balancing act between riding fast and being conservative enough that you have the necessary energy left to finish the race strong. By pacing your race well, you'll be able to catch riders that are fading. Pacing also minimizes the risk of mishaps.
To ensure you're pacing correctly, ride the first third of the race a little easier than you think you should, which will ensure you save enough energy for the later stages. Experienced racers use the lap function on their GPS device to monitor their pace lap-by-lap, which is a tip that can serve riders of all levels.
Ultimately, though, ride your own race and be sensible enough to let faster riders go if their pace proves too fast for you to hold.