For long term storage, by far the best solution is to use one of the vapor phase inhibitor (VPI) products. There are several forms available, but the best is VPI-impregnated seal-able plastic envelopes. These are available for both rifles and handguns. Available also are VPI impregnated wood chips which can be placed inside seal-able plastic envelopes or inside gun safes. Brownell's has these.

For shorter-term protective storage, I think a paste wax coating is as good as anything else you can use.