If you look at the Test Flying section of my [link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=1031]E-flite F-15 Eagle[/link] review, you will see one way to balance rotors. Using a good balancer like the Dubro Tru-Spin Prop Balancer, you simply sand the blade tips until the rotor no longer favors the blade. You can also dab a small amount of clear nail polish (or spray paint) on the opposite side to add some weight.

Good luck!