Your battery needs to be both padded and installed in a manner where it will not move around during maneuvers or hard landings. Same goes for the receiver, it is lighter than the battery pack but can also influence balance to a degree. Keep as much EXTRA weight out of the plane as you can. Remember, the further an object is from the cg the greater the effect it will have on the balance. This means that 1 oz of lead under the tail will have the same effect as 10 oz right behind the servos. (Not exactly correct but meant to give you the general idea.)

Bruce[