Here's my take...
Of course, pay attention, keep windows up, doors locked, and keep a safe distance from the car in front of you. Expand that to "always have someplace you can maneuver to," and that goes for non carjack situations, too.
Assuming someone has chosen you as their victim, determine if you can give them the car and get the hell out, and if so, at least consider that. I carry two keys to both our cars, regular and "valet" -- the "valet" keys are on a quick release and I *always* drive with those, so given the option, I can snatch the other keys and hop out of the car. I do that for several reasons; one, for valet parking
; two, so I can run inside a stop-n-rob and lock my family in the car with it running; three, in case I want it to appear that I have removed the keys from my vehicle; four, so I can abandon the vehicle quickly and not leave housekeys behind.
I CCW but if I'm driving my POS VW the guy might be doing me a favor. If I'm with my family or in my wife's non-POS Jeep, probably a different story.
Remember that a $500 car insurance deductible beats the snot of out a $500 health insurance deductible, if the perp gets even a little lucky.
Especially for women -- the line of "fight/don't fight" is this: if someone tries to take you somewhere, fight, because you are most likely fighting for your life.
Anyway, assuming you're being carjacked while you are stopped and in the vehicle, lean on the horn and don't let up unless you need your hands/elbows. The idea is to attract as much attention to the situation as possible. Along those lines, I have to agree that intentionally bumping another car is a *good* idea. The intent is to make lots of people stare, and hopefully the person you hit will even get out of their car. Witnesses are bad mojo to crimes in general.
If you can get out of the car, do so, because you are at a location that you at least chose to drive through. If you are kidnap/carjacked, WRECK YOUR CAR. Slow down and drive into a phone pole or something. No one wants to steal a wrecked car. Hopefully the perp will get out and run. Refer to advice re: fight/no fight above -- if you're driving and the perp is telling you where to drive, he is taking you somewhere. Bad. Escpecially if you have little ones in the car. I'm not saying drive it off a bridge, just smash something. Kids have to be in car seats anyway, right? You're wearing your seat belt, right? He's not, right? Hmmm...
If you're not in the car yet, consider these obvious suggestions -- ensure that your car has a remote keyless entry system, and that the first "press" only opens the driver's door, the 2nd press can open the rest. Obviously check the backseats/etc before getting in, and lock the doors the second you're in the car (hell, I do that anyway, and I carry). If you can reach them, consider not buckling the kids in until the door is locked. Don't park in stupid places, and if you do, don't walk out there alone.
As has been said, if you're driving and someone "bumps" you, don't stop someplace secluded. If you're not sure where to stop, call 911 and ask them. Better safe than sorry. If you're pulled over by the police and feel suspicious (as in fake cops), turn on your dome light, your hazard lights, slow down (less than the speed limit), and wave your cell phone in your rearview mirror before calling 911 and asking dispatch if you're really being pulled over. Your neighborhood LEO should understand the meaning pretty clearly.
For those of us who carry -- if you witness a carjacking, at least consider that if there is a owner in the car, you just witnessed a kidnapping, and if the owner is female, you just witnessed the beginning of a rape. How should you react to a rape in progress?
I disagree with the theory of avoiding eye contact. Look around, look right at people. Look at their face, clothes, hands, friends, etc. -- look "observant." You don't "look like a victim" when you do that. If someone gets nasty with you for looking at them, you may have pissed them off just by breathing their air, you never know.
Did I miss anything? Anyone disagree?