I read someone who said it, I'll say it too, make your bike ugly. I have a baby seat on the back of mine but most of the time its empty but it adds a lot of ugly to it. Also having large white wall tires on a bike that looks meh adds to the ugly. It doesn't take much for the bike next to mine to look like a better, more salable target.

Few pictures of it are in the trailer threads...

Oh, a trailer helps too. I had someone come into our yard and steal my boy's 20" bike. He didn't steal either of my bikes, nor my daughter's full suspension 26" bike, mostly because I'm guessing they wanted out fast and both my bikes were attached to trailers and surrounded my daughters bike. They took the quick and easy...that was my guess.