I would have to agree with Bruce here. How much movement are we talking about?? You will have a little bit of movement in a properly mounted servo because of the rubber grommet, but that's good because the rubber is doing it's job of isolating the servo from vibration. But if you are getting a lot of movement then you have something seriously wrong with your setup. My guess is that you are trying to move the control surface past it's limits. Once you hit the limit of movement something has to give as you continue to push against it. Right now it's only the servo moving, but eventually the control surface will tear lose from the plane. If this is indeed what is happening then you need to change the position of your pushrods in the control horns to adjust the amount of travel of the surface.

If you have flexing or moving of the servo going on it's because you have something wrong with your setup. You can't just look at the pushrods, but you have to look at the entire setup as a unit. The servo, servo arms, control horns, pushrods and control surfaces need to all be looked at to find problems like this.

Ken