Simply storing a firearm in a closed case is a bad idea since moisture in the air can condense on the firearm and cause rust. As curt mentioned, a desiccant or treated paper can prevent this.
If you use a case with foam, the foam can also act like a sponge and soak up any lubricant on the rifle – after which it rusts.
Depending on how much trouble you want to go to, you could coat the AR with RIG grease (which is similar to cosmolene), but it's really messy. IMHO, RIG is too much trouble if you’re only going to be storing the AR for a year and you’re not going to be storing it in a hostile environment (say a damp basement). Brownells sells RIG.
If it was me, this is what I’d use: [url]www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=1116&title=TRIPLE+TOUGH+RUST-BLOX+STORAGE+KIT[/url]. Incidentally, you can get the components of this kit separately – if you only want to store one rifle, this might be a little cheaper way to go.