well, the first thing to do is to have all the wheels Road Force Balanced. (Road Force Balancing applies a Simulated load as if the wheel was still on the car, giving a more accurate balance) and make sure the rims are as clean as possible, as clumps of dirt and Asphalt/Tar Covered Stones will also cause the wheels to be thrown off balance. another thing to check is if you have any bent rims, as that would throw the balance off as well.