Security of bike on trunk mounted bike rack

well...personally I would throw one wheel (and seat if its QR) inside the car locked up and lock the rest to a post or similar nearby. Not sure you can get too safe while on the rack, I've heard of folks taking the whole rack to get the bike.

We throw a cable lock around two frames and the rack, not really a lot of security but it prevents those that might be a crime of convenience, this way who ever would try to take the bikes would also have to take the rack as well all as one unit. We only leave it unattended for short periods of time in what we feel like are pretty safe areas as well.

I have a hitch-mounted carrier, so I loop my cable through the hitch and the frame of the bike.

You might be able to find somewhere under the bumper where you can loop a cable/chain to lock the frame/wheels. Like others have said, remove any easily removed items (seat, computer, bag). You might also loop the cable through the rack.

I have a trunk mounted bike rack, but when I do a brick, I take the front wheel off of the bike and put the entire bike in the back seat of my car.  Sure, it kills transition time, but if I'm really concerned about a fast transition, I do the brick from my apartment where I can just run in, leave my bike, throw on the running shoes, and take off.