I used white engineering reflective tape. They'd have to be very dedicated thief to notice to the Deore XT components.
Plus, the reflective tape is very visible at night.
The thing is people aren't stealing them to always sell them. Sometimes it's just easier to quickly grab a bike when the thief needs to get across town for some other reason. When they get to where they want to go, they just ditch the bike. Sometimes I think some people just want to steal a bike for no real reason.
In Seattle, I've seen gigantic piles with hundred of bikes in them just sitting in a homeless camp. They clearly aren't selling those bikes, as they've been sitting out in the rain for months, or even years now. And I guarantee those homeless camps aren't full of multi-thousand dollar bikes.