Here is a tip based on one I picked up from RCLibrary forum for lateral balancing wings. While this is highly applicable to gliders due to their long wings, it may work for all planes.

Before you cover your wings, join any parts that come apart to create the wing unit. turn it over and balance it on a string that runs down the center from the front to the back. Tape the string in the center if it will have a tendancy to move.

While inverted the wing will tend to drop to the heavy side. Add weight near the tip of the wing in the form of brads, thumbtacks, lead shot or lead split shot fishing weights until the wing balances.

Remove the wing from the balance, temporarily attach the weights and then retest the balance to be sure the first measurement was right. Now permanantly set the weights. Be sure they will not move over time due to hard landings or heat build-up.

http://rclibrary.com/viewtopic.php?p=413#413

Do you have another approach? I would love to hear it.

Happy flying.