1: Know the course
2: Have a plan
3: Be flexible
If at all possible, I love to preride the course. This helps to know the parts of the race where I may excel and where I might be better off settling into the pain. Are you a climber? Do you have mad bike handling skills? Figure out your strengths and use that to make your plan.
Lets take this race on Saturday for example. The course profile looks like a 5 mile hillclimb (it's a pretty small % grade not exceeding over 5% for longer than a hundred feet), followed by a 5 mile downhill ride to the beginning of the loop. The Sport and Single Speeders will do two of these laps.
My race plan usually starts the same for every race.... Go 110% for the first 10 minutes or so. This either discourages the other racers to the point where they will slow down so they don't explode, or if the guys are faster, it at least gives them the impression that I could hang and that I might just be seconds behind them the whole race.
For the rest of the climb on the first lap, my plan is to settle into a fast pedaling cadence without burning up my legs to bad. I want to maintain a hard pace up the hill, but still know that I have another lap to go. The next part is huge...... When you get to the top of the hill, DON'T STOP PEDALING!!! Just keep going! Never let off the gas and your competitors won't be catching up. Stay calm and smooth on the down hill section. Generally in a Cross Country MTB Race, descending is NOT where a guy will WIN the race, but it could be the part where you can LOSE the race. Don't crash!
Lap two will be harder than lap one. I intend on putting every ounce of energy into getting to the top of the hill faster than anyone else.... "why would you do that when there is still 5 miles left in the race?"..... Like I said before, nobody will win this race with descending skills. Just maintain a smooth (smooth is fast!) style down the mountain and always be ready for a possible sprint to the finish.
Lastly, be flexible. A crash, a flat tire, your chain falling off, getting stuck behind someone in some tight single-track could ruin your flow. Race your race and don't let anything get to you. Brush off the dust and think "be smooth bro, be smooth". You will get your groove back and you may actually find yourself going faster afterwards. Sometimes it's more fun to chase down the guys or girls that just passed you and blow them away on the next climb :)