What Minnflyer said. The rubber needs to be on the top and the bottom of the mounting flange. The other huge mistake that made quite often is that people will crank down the servo mounting screw as tight as they can. This is a sure fire way to servo failure. The screw needs to be tightened just to the top of the rubber grommet, and no further. This is done to ensure that the rubber grommets absorb vibrations from the airframe. If you over tighten it the rubber can no longer absorb the vibrations and they are then transfered directly to the servo, which will lead to servo failure.

Ken