The most protective method is the one now used by the military and industry to store rust-able firearms and equipment.
This method uses a special "hard" plastic bag and a special "vapor paper".
The plastic bag is made of a special plastic, that unlike ordinary Zip-lock type baggies, won't pass moisture.
The Vapor-Paper looks like brown wrapping paper. This paper gives off a vapor that drives air from the bag, and surrounds the metal with a vapor barrier that TOTALLY prevents rust.
A gun sealed in one of the bags with the paper will totally prevent rust for a minimum of 10 years.
The military uses this method because unlike the old method of coating the gun with cosmoline, all you have to do is clean the gun before putting it in the bag.
When you need the gun, all you have to do is take it out, and start using it, without a lengthy degreasing to get the cosmoline off.
You can buy the bags in both rifle and pistol sizes, and the paper in sheets of various sizes from Brownell's. http://www.brownells.com
If your primary concern is rust, this is the best storage method ever found.
However, DO NOT use ordinary plastic bags, use the special ones sold for this purpose.
I would recommend that you get a can of #2 Rust Prevent from Brownells. It is a watery liquid, not any kind of gel/grease; just put a little on a rag and wipe down the gun. It will dry and you can't even tell it is on there. Gun will be protected for at least a year. A quart can of this stuff used to be around fourteen dollars, and will last forever. I bought mine back in 96, split the can with a friend, and still have enough to last forever. Great stuff, and should you handle the gun, it protects against fingerprints, etc. I have wiped down firearms with this stuff, and left them down in a damp basement, or out in the barn, never had the slightest sign of rust. Cannot praise this stuff enough. Best thing I have found besides a heavy grease like RIG, or Cosmoline. Best of all, you need do nothing when you are ready to shoot the gun. Just load it up and go.