The best I've ever used is called K2R. It's intended use is as a carpet cleaner, but will do wonders for getting fuel out of balsa. You can get it at Wal-mart. Spray it on and let it "soak" for a few minutes then brush it off. You will definitely need to do this several times to get all/most of the fuel out. I've found that heating the balsa between uses will help force more fuel out of the balsa.

Hope this helps. Good luck