Like one of the posters, I use a Saris Bones trunk mounted rack with a steel cable wound through the wheels, frame and rack, secured with a Master padlock. It might slow a thief down a bit, but if they really want the bike all they have to do is cut the straps holding the rack to the car and take the whole thing. 30 seconds and they're gone. At least they would have to carry it rather than just ride away!

I even pull the rack off the trunk when I leave the car at a trailhead because there is no practical way to secure it.

All a determined thief needs is a good bolt cutter and a couple minutes and almost any bike can be stolen. The best we can do is not make it real easy for them.