Staying off severely rutted or muddy trails is another. Know where you're riding. Wear a helmet. This is the best way to get involved with the trails you ride. Most clubs hold regular recreational rides and trailwork sessions.
Some clubs promote races.
Nearly all clubs have an abundance of knowledge about great trails to ride that aren't far from home. IMBA is the worldwide organization that represents mountain bikers. We work to keep trails open for mountain biking, we promote responsible riding and volunteer trailwork, and we strive to improve the sport's image. Sometimes this work is building new trails. Sometimes it's repairing eroded paths.