This may come under the 'who didn't know that?' department, but in case you haven't come across this product, it is highly recommended. *It is basically lanolin in a can, and sprayed on anything metallic, it will prevent corrosion or rust for a very long time. *If the piece is under cover, it will do the job indefinitely. *Someone else has already noted Petro Tape, which is wax impregnated cloth and it is also a great product, but where it isn't practical to wrap the particular item you are trying to protect, you can spray it down with Fluid Film. *For example, black steel bolts on the engine mounts, the underdeck components of the windlass, etc.
The only disadvantage is that it creates a lanolin film, so you don't want to use it on components that you have to come into contact with.