The main cause of damage to the trails in Bristol is riders using them in wet conditions. If it's rained recently you might want to give them some time to dry out again (a couple of days, or longer if it's winter). Don't widen the trails by riding round puddles - there's a good chance you are avoiding a patch of trail that has already been armoured, but which has collected a small amount of standing water. Riders can also erode the trails by locking their brakes, so keep it smooth rather than skidding on downhills or before corners. Finally, this is obvious, but please come out and lend a hand on trail days.