The CMBA is all about getting more trail for everyone to ride. However, not all trail is good trail. The CMBA is committed to the creation of sustainable trail.

The International Mountain Biking Association came up with some guidelines in 2006.
http://www.imba.com/resources/trail_building/sustainable_trails.html

Check the link for the nitty-gritty. The key points to consider when building trail are:
1) limit environmental impacts
2) keep maintenance requirements to a minimum
3) avoid user conflicts

How do you do this?
1. Do everything you can to keep the water off the tread, and users on it
2. Build on the contour and use frequent grade reversals - surf the hillside
3. Follow the half-rule: A trail's grade shouldn't exceed half the grade of the sideslope
4. Maximum grade should be 15 percent (except for natural or built rock structures)
5. Average grade should stay under 10 percent (with grade reversals)
6. Route trails to positive control points (viewpoints, water, other attractions)
7. Use bench-cut construction, and excavate soil from the hillside
8. For reroutes, reclaim old trail thoroughly - the visual corridor as well as the trail tread
9. For highly technical trails where grade will sometimes exceed 15 percent, use natural rock, rock armouring or other rock features to add challenge and improve sustainability.

The long and the short of it is that to build sustainable trail you need to stay off of the flats and don't run your trail straight down the hill. When you build on the flats, you get the bogs of Tom Snow. When you build on the fall line [straight down], flowing water will cut a giant rut in the middle of your trail, and invite the wrath of Parks and SRD.