The best way I have ever found is to use RIG UNIVERSAL GREASE inside and out. Apply with a patch & bore jag in the charge holes and barrel, and use an old shaving brush or similar on the rest of the gun. I also take the gun down and do the inside as well so I have 100% coverage.
Wrap the re-assembled greased gun in wax paper, then again with heavy duty aluminum foil. Now you can stick it in the original box and store in your safe. It will NEVER rust! BEST preservative I have ever found and I just de-greased a stored gun after having it "RIGED" for 30+ years. That particular gun was RIGED by my Dad and I and stored in his un-airconditioned basement for 30+ years. He passed away last March and I took the gun and cleaned it up------good as new!!
By the way, RIG stands for :
Rust Inhibitive Grease.
Chief38