Originally Posted by pdlamb

I can't remember who first suggested this. Assuming you have a chain tool and have (or can get from your LBS) an old chain, break the chain into an appropriately sized piece. Loop the chain through the seat rail, then down under the seat tube/seat stay junction, and reassemble. Chances a prospective thief will have a chain tool are vanishingly small, and chains are tough to saw or cut. If you're worried about the frame, run the chain through a piece of old inner tube first.