Always stop the car so you can see the wheels of the car in front. You will have left enough distance to drive away without having to reverse.
A car cigarette lighter is a very effective weapon, push it in if you feel threatened so it is good and hot if you need to use it.
In certain circumstances, I have carried two wallets, one of which contains a small amount of cash and some of those cr@p cards you get for membership of video clubs etc. No great loss to hand that over.
If you are going to carry a weapon, make sure you know how to use it, because a criminal is very likely to know how to relieve you of it and use it on you!