as others have stated, not much you can do to stop theft completely
if stolen directly from inside your home, no different than taking your jewelry or electronics
when I ride where I live, I rarely leave it unattended
When in the city take my 9mm hardened steel chain with a mini-U lock and a second U lock or lighter chain
- always secured to fixed object
- always secured in a busy location
- never leave overnight
- recognize a "pretty" bike will draw more attention
I seem to be hyperparanoid with bike security in NYC, more than anything else (lived there for 10 years)
getting a bike stolen really sucks