I do lots of trail building and maintenance, granted the work I do is more for hikers, not so much for MTBers. Anyhoo, my plea would be to be consciuos of how you cut into soil or remove vegetation while making the trail. You don't want to do anything that might accelerate or promote erosion. Think a little bit about, and maybe do some research into how you could do something to ensure that what ever you do to make it as sustainable as possible. I do agree with DrGonzo that it is probably smart to confirm who, if anyone, owns the land. It is one thing to ride on it, a whole other to alter the character of it. I do realize, however, that a lot of people could care less about stuff like this.