Security - junky bicycle

Ride a junky bike, if you're not picky about the appearance or performance of your ride. Hopefully, a thief will move on to a better one. This is not 100% effective, so use locks too; thieves sometimes steal even the worst bikes.

Thieves usually are male, and will be less attracted to women's bikes. If you are willing to ride a women's bicycle, buy an old used one. Paint it hot pink. Put glittery heart, unicorn, and Barbie Doll stickers on it.

Or just buy a junky looking bike that still suits you.

Either way, you can make it look still worse:

I'm not sure whether defacing a good expensive bike will work. Thieves might recognize a good bike just from the shapes of its frame and parts. If I owned such a bike, I wouldn't want to ruin its appearance anyway.