I use a cable lock with a combination. It possibly isn't the most secure lock in the world - but it should put off the opportunistic casual thief. I reckon that to stop a really determined thief I'd need more or heavier locks than I am prepared to carry. Also use a cheaper bike for my errands/commuter bike - the purpose of that being two fold - hopefully it is less likely to get stolen and that it wouldn't be such a big deal if it was (although my emotional attachment to it and its replacement value went up about 50% when I got new pedals for Chrsitmas)