The Four Commandments of Cable Routing:

1. The handlebars must be able to turn as far as they can in both directions without being limited by a cable pulling taut. Instead, the turning limit must be set by the handlebar bumping into the top tube or by the brake arm or reflector bracket bumping into the down tube.
2. No wrong direction bends (For example: as the rear brake cable leaves the top tube and makes the bend down toward the caliper, it should make a smooth transition from parallel to the top tube to parallel to the seat stays. If the cable bends up from the top tube before bending down toward the seat stays, it is probably too long. If the cable curves out past the caliper, then bends back at an angle more vertical than the seat stays, it is certainly too long.
3. The bends that cannot be avoided should be made as wide (gradual) as possible,
4. Cable housings should be as short as they can be without violating the above rules. If a housing is attached to a part that moves, test it at the extreme positions it will take when in use. Apply a brake; shift a derailer to the positions where the cable is tightest and where it bends the housing the furthest in each direction.