You are well within your rights to be very annoyed and show this in whichever way you normally do. This could be by beeping loudly, flashing your lights, shaking fists, swearing, rolling your window down and shouting something withering - anything. For some excessivley sensitive souls, it could mean tailgating the offending car for a few miles while glaring hotly at them in their rear-view mirror shaking your head and mouthing 'you fool' every time they glance worriedly at you.
You are a survivor in life's little struggles. There may be a good reason why this
Another thing is, will it turn out that you know the person, or that someone you know is in the car behind? If you live in a small town or city, then the chances of this are highly likely. Remember, real life, walking around, driving etc, is a lot like social networking. And just like social networking, never, ever do anything in public that you wouldn't want to get back to your mum.
Finally, although you might be a strapping six-foot lad, is the person in the car who just cut you up, actually an axe-wielding giant? Always be careful whom you offend on the road.
So, how do you survive being cut up?
If you can, let it go completely. Cultivate an atmosphere of calmness in your car so that when little things like this happen to you, you can just shake it off. If you are stressed, thinking about your horrible day, have loud music blaring or arguing kids in the back, then just one more kick could light your fuse. But if you are perfectly at peace and stable listening to your favourite thing, then this will be just as if nothing happened. You can just brush this annoying little person aside as if they were a passing fly.
That is fine if they have turned off or are in another lane. Sometimes someone has cut you up and then you have to follow them for miles seething and fuming behind them. Usually they are also in a better or newer car too (...the effing, bleeping...calm down, breathe). The only thing to do here is to pull up somewhere for a second, or let another car in between you and them. Turn off and go another way.
You will find, that as soon as that car is no longer within your sights, then it is gone from your mind. It can become a funny story that you tell when you get home, nothing more, nothing less.
Don't forget, someone who has cut you up really might have a pregnant woman in the back or be rushing home to their dying dog. These things do happen too. Give them the benefit of the doubt.