I carry a computer security cable thing which is long enough to wrap round the bike many times and be a bit of a pain to remove but I know could be snipped very easily. And a meaty padlock that can work as a disk lock on the back wheel (an extra bonus of discs) which means any thief is going to have to carry the thing. These only exist so that if I end up with an emergency that means I need to dive in to a shop for 2 minutes (batteries for lights, food to deal with a bonk, etc.) then I can do it. My hidey hole at work has two sold secure locks in it permanently, one a proper chain and the other a U lock. Plus is behind two doors with very few key holders. When I get home they live in the house. It is possible having a bike stolen has made me over careful.
We've just been to Amsterdam for three days and cycled to and from the pub we were in. I had 5 locks with me which probably cost twice the value of the two bikes we locked with them. Then again they were our two cheapest working bikes (Giant Boulder and 80's Raleigh racer thing a friend gave us two weeks ago rather than throw away).
Ask yourself, am I paranoid enough?