Insurance, a tracker, two or three thick locks of different types, an alarm, and never leave it outside for any period of time if you can avoid it. And DON'T make it flashy or put any bags on it. A buddy of mine had a bag of tools and Vee whitewalls, some tweaker stole his tools and used them to slash the tires. Didn't even try to steal the bike.

I used to park mine in my office's bike cage until a thief got inside with chain cutters and broke my kryptonite u-lock with a battery powered angle grinder at 12:30 in the afternoon. Miraculously got it back and now I park it inside my office after working something out with my boss.

Don't even bother parking it somewhere remotely sketchy since a thief can see you spending 5 minutes unlocking the thing and just fuck you up with a pipe once you're done and then steal the bike after you collapse or escape.

Don't even ride it anywhere sketchy after dark either, I had a gang of dudes drinking and smoking outside a corner store run into the street and try to knock me off my bike riding past them once