I will be doing my first MTB race this Sunday. Any tips or tricks? Are there any unwritten rules of the trail?

I do not plan on winning or placing, mostly just want to finish. I did my first century on the road bike about a month ago and did my first road bike race(88.5 miles) last Sunday. I will be entering the Citizen Clydesdale class and the course will be only one 12 mile lap. I've been back into riding for about 4 months after almost 20 years off bikes.