What Jetmech05 said. Do everything you can with the existing weight first. I have even sliced a firewall and moved it back to help with nose heavy. Move the engine as far forward as possible on the mount if it is tail heavy.

With ARFs that have rear servos I like to tape sheet lead to the base of the servo case (two sided foam tape and then wrap the case & lead with Vinyl electrician's tape or a Nylon cable-tie or three). This works great with aileron servos to laterally balance a model, too.

On my Contender 60 with a heavy four-stroke I ended up using #2 bolts to attach sheet lead strips under the horizontal stab on each side. Since then I have built hatches into the tail of kits and fitted a ply "sled" with bolts and "T" nuts that I can remove with provisions to bolt lead to that sled. Makes for easy trim adjustments. Remember that the further out on the extremes you add lead the less you will need to add. A heavy brass acorn-shaped prop nut instead of a plastic spinner, for example.