Just ask pro triathlete Wes Hobson. I was too cocky and descending too fast. I hadn't yet acquired the technical skills that seem to improve tenfold each time you ride a mountain bike for the first 10 rides. Well, for one thing you won't have to be in a hurry to develop race fitness as he was. So spend the first week or two on very easy trails.
Fire roads are often a good option since they're generally far less technical.
This will allow you to get a feel for the bike with easy challenges while staying in control. When you're ready to graduate to somewhat tougher trails try riding laps of a short course. The repetition will help you master a needed skill quickly. Another way to develop off-road handling skills is to ride behind an experienced mountain biker. Watch and emulate how he or she handles the technical sections.