Go really hard at the start in order to get in front of the folks that hold the whole group back on the technical single track. Stick with the lead group as long as possible until lactic acid builds to much (in the red zone here) about 8-10 minutes. After this I allow the legs to slightly recover just enough not to blow up. For the rest of the race I basically ride hard as possible being on the verge of cramps the last 3-5 miles. So basically my only strategy is ride hard as possible without cramping. If I'm feeling really good I'll hang in 3rd to 4th place at the start and then put a move on about 2-3 miles in and keep the hardest pace I can until I know I've put some distance on the others, then back off to a maintainable pace.
For me when I'm race fit..
Be in the top 3-5 into the woods/singletrack if you think your top 3 that day. Otherwise, the inevitable guys that are slow in the tech(but strong on the roadbeds) will hold you back and can make the difference from getting on the box..or not.
Hang on best you can without blowing up for the first couple/few miles. The pace 'will' let up some after that. Other riders typically/hopefully will drop off the pace during this process. In the meantime, you've hopefully gap'd other riders that you otherwise finish around, because they're back there wasting time getting around slower riders.
Try and recover best you can on the descents but while not letting up on your best downhill pace possible(easy to say, but when your oxy. dep'n and at max HR when you crest the last climb, you have to make yourself focus at these times, or a hard crash may result). A lot of riders let up at this time(see next paragraph below). Don't.
If you have good downhill/tech skills, if at all possible..always always get by the next rider(assuming hes with-in striking distance) before the next descent starts. Otherwise your stuck at their pace and your tech riding advantage has been basically neutralized for that descent
Go as hard as possible for the rest of the race(unless your in the lead and can tell if no one is catching you) and pass asap as you catch anyone, regardless of class(never 'settle in' to another riders pace, because as soon as you do, riders ahead are now possibly getting away..or your poss. now being over taken).
Perfect pace/effort for a given fitness level is almost cramping the last mile or two, but don't. Then fall out at the finish line.