For the love of god, take the strings off I left my guitar for a couple of weeks, and the strings were god damn awful when i got back. Rust from them had spread to my pickups apparently, and i just plain dont like that! Ill tell you now, i know nothing about properly storing guitars, but my best guess is inside a case, with one of those moisture collecting pouches that you get in shoes etc, and probably flat. Storing upright might put a tiny bit of pressure on the neck, but if its for a long time, who knows what could happen with that constant tiny bit of pressure.