Really don't like the brake disk locks, since trying to ride off with one attached tends to damage your bike, your body, and your pride. They also don't prevent three or four big suckers from just picking the bike up and putting it in the back of a truck. Hauling around a piece of logging chain and a huge padlock also sucks - uses up too much space on a bagger, and damn near impossible on anything else. Taking along your favorite Doberman might work, but you would probably need a sidecar for the dog, and the thieves might be bright enough to bring a chunk of meat with them for a diversion. So far, other than staying with the bike while armed with your weapon of choice (9mm, .45, 12 gauge, etc.), I don't think I've heard of any foolproof method. I just lock the bike, set the alarm, and park in an area with lots of visibility and foot traffic so anyone hoisting the bike into the back of a truck would be a little to obvious. So far, it's working. If it doesn't, that's why I have good insurance.
Jerry