One tip I learned from driving my F250 Crew Cab Diesel pickup for 100,000 miles, if you have something on the back of your car (or in the bed of a really big truck) back into whereever you park. Back close enough to a wall or car behind you so the tail gate can't be opened or a bike can't be easily removed from a rear rack.
Doesn't anyone else lock their bike to the rack? I have a rack that folds down and I run a cable through the bikes and attach the cable plus lock to the rack in a convenient hole. I also lock the rack to the hitch with a hitch pin lock. I know there are racks that hang off the back with straps and can't be locked onto the vehicle...but you could at least lock the bike to the rack and slow up the theives that way.

Karen