One of the best ways to get started is to fill the keg with very hot water. This will loosen the glue and stickers enought that you can get much of it off with a scrubbing pad and some mild detergent or oxyclean. Don't use anything too hot (maybe 130 max) or it might loosen the glue holding on the rubber bottom as well. (If it does, they can be glue on pretty easily).
For more stubborn residue, any vegetable oil or pam spray will help.