I'm a trail building junky. I haven't done it near enough because I try my best to do it the right way. Legal, community involved, everyone can ride it, proper property management notification and of course, sustainable.
Just like any other labor based skill, trail building is one part education, one part research, and another part experience. I try to dig up as much education and research as possible before I lay down the trail.
Here's a few links of interest from the west coast guys of NSMB.com:
NSMB: A Culture of Building by Morgan TaylorNSMB: Men Behind the TrailsNSMB: Rocks and Gold by Morgan Taylor