Something that I don't think has been mentioned is make sure you buy a helmet! Try a bunch of them on, make sure they don't rub weird, etc. Make sure it's comfortable.
Two times a helmet has protected me:
1) I managed to crack a helmet shell mountain biking while riding a relatively benign section of trail that I'd ridden probably 30+ times before with zero incidents. There was a little exposed section of roots that I caught just wrong, tipped me sideways and I ended up bouncing my head off a log on the ground.
2) Riding a sort of narrow bridge over a wet spot and culvert, and managed to drop the front wheel off the boardwalk just before the culvert, and proceeded to go over the handlebars face first into the dirt with the helmet probably helping prevent me from breaking my nose. Lessons learned: Wear a helmet and don't look to far ahead down the trail!
Two times a helmet has protected me:
1) I managed to crack a helmet shell mountain biking while riding a relatively benign section of trail that I'd ridden probably 30+ times before with zero incidents. There was a little exposed section of roots that I caught just wrong, tipped me sideways and I ended up bouncing my head off a log on the ground.
2) Riding a sort of narrow bridge over a wet spot and culvert, and managed to drop the front wheel off the boardwalk just before the culvert, and proceeded to go over the handlebars face first into the dirt with the helmet probably helping prevent me from breaking my nose. Lessons learned: Wear a helmet and don't look to far ahead down the trail!