depending on where you live and terrain and such, water erosion is one of, if not your biggest, enemy (other than the jackwagons Chris F above mentioned, who make side trails and short cuts around obstacles). so try to keep your trail as "invisible" to water flow as possible. a good way to do this is take a tennis ball (from out of some BMX kid's spokes) and roll it down the hill. the tennis ball = a water molecule, and you can see where water will flow. build your trail so the water runs over it, not down it, creating a rut which, over time, becomes an ugly scar on the earth. water bars and such help.