Right now I use a Kryptonite New York 3000 U Lock and a KryptoFlex Cable lock to lock the rear wheel. I've made myself reasonably paranoid by watching a bunch of youtube videos of bikes getting stolen. It's amazing how easy they make it seem. I have the advantage of being able to bring my bike into the office at work and keep it inside my apartment as well. I only really use them when I go do some light groceries. I do plan on getting a small mini lock just to make it a little more difficult for whoever is eyeing it. My bike is worth about $500 and even though it hurts to buy a hundred dollars or more worth in locks I can just imagine regretting not had in the future.