Bare metal is prone to oxidizing but a painted surface is not. Try painting any bare metal you want to protect. This should prevent any rust from forming. Paint also can be an effective method of stopping a small rust problem. Paint the area where rust has begun to develop. This should stop the spread of oxidation.
This method may seem counterintuitive since water or moisture is what helps rust form. The trick is that periodic washing of the metal you want to protect, such as the undercarriage of a car, will keep a buildup of dirt from holding moisture against the metal. Try a pressure wash with a hose and make sure the metal is in the open air to dry completely.
Bluing is a coating process used on steel to prevent oxidation, much the way paint protects. Bluing, however, is used primarily on guns. The chemical finish of bluing gives guns a glossy cosmetic look while preventing rusting and chips or other corrosion of the gun finish. The coating is an oily chemical finish that displaces water from the surface of the steel.
Plating is a solution to prevent rust, but it is somewhat labor-intensive and can be expensive. Zinc-plating is a process in which iron or steel is coated or dipped in a hot zinc mixture that dries and hardens. This is also called galvanized iron or galvanized steel. Tin-plating is similar but used only with steel. A steel sheet is dipped in a hot tin mixture, much the same way as in zinc-plating.
The method of controlling humidity is effective but not always possible. This process depends on where you live and what metal object you want to protect. If you can keep humidity levels low around that object or keep the object away from pockets of stagnant water, rust is unlikely to be a problem.