AXA defender Ring lock + 140cm lockup chain ..
I like a lot..
helps that frame has fittings for mounting the RL.
Chain is wrapped around seat post
and across the mounts for my Carradice saddle Bag.

http://clevercycles.com/products/accessories/locks/#_

Big plus, lock is on the frame, quick stop in and out of the store ?
then the hasp of the lock goes around the back wheel, so no one can roll it away.

next level of security , chain around front wheel, then a fixed object,
and pin on the lock , plugs into a catch on the lock..
take the key out.. , ride with it in..
just don't lose the key in the store..