Try to keep your wheels on the dirt, avoiding the green vegetation to the sides of the trail. Sometimes the trail is very narrow, providing an opportunity to improve your riding skills. The number two rule is don't encourage erosion . This could be seen as even more basic than rule number one, because vegetation helps prevent erosion, and staying on the trail preserves vegetation, thereby helping to prevent erosion. In any case, the thing bicycles can do that speeds erosion most is skidding.
Many Web sites address the issue of erosion.
This is an academic study with photos and other documentation. Because the entire report is on a single page, it is slow to load, but you can read it while the pictures are loading. This is part of an individual's home page. It clearly identifies the main issues. Repair Trees which have fallen on the trail should be cut back to allow clear passage by all trail users.