Just to add my 2 cents, the larger the ground wire the better and the best technique is to "home run" grounds to the battery negative. That is each ground runs directly to one central point and not running a ground to one motor case and then using a short jumper. With D.C. it goes through the whole cross section of the wire thus the more "cross section" the better. One technique is to have a large screw / bolt to run the grounds to and then one heavy wire to the battery. In the case of R.F. interference such as when you are also using a transmitter the use of ferrit beads at the ends of the wires will help. There is also copper tape you can use to form a "shield" around servo's, etc.