I was suprised to find that in XC it was considered bad form to fight your way past people on the technical single track sections.
If you are on the slower side be aware towards the end of the race the fast guys will start to lap you, It was always considered bad form not to get out of thier way.
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I've raced XC for years, and it's perfectly ok to pass riders anywhere you can, just don't knock 'em off the trail when you try. Always try to communicate your intentions to the rider you want to pass. Unless in a sprint section most riders will get out of the way (if safe to do so).
Also, watch out for folks lapping you as they will ask to pass you. But don't be forced to move out of the way if it's not safe and you can always let the rider behind know when they can pass.
If you can get a preride in before the race then I recommend it so you get an idea of how tricky some of the sections may be.
The start can always be interesting with a lot riders sprinting off line to be first to reach the single track. As your first race I'd stay back and not try to get into the sprint at the start. Plus a lot riders blow up after the first sprint and slow right down, making it easy to pass 'em.
Depending on how long the race will be, you might want to consider warming up before the race. Keeping pedaling your bike until you have to line up, so legs are good to go at the start.
Don't try anything new on race day. I don't know how long you've been riding, but don't try new shoes, tyres, pedals or a whole new bike on race day. Stick with what you know and what works for you.
Smile for the cameraman when you see him.