Preparation for a track day or race

Your car:

Tyre pressures
If you only do one thing before a track day, try to get the pressures right. This is a bit of a dark art, and many drivers are secretive of the pressures they run. At the very least, match the manufacturers guidelines, and as a rule of thumb, increasing by 5-10% should provide a better turn in and be generally more suited to track conditions. Experiment here, and take notes – eventually you'll find pressures which will give you a good compromise between grip, predictability at the limit and turn in. Pressure increases with temperature, so bear this in mind if you'll be doing a large number of laps - if you're doing short runs, you can probably get away with slightly higher pressures. Rubber compounds also tend to get softer and provide more grip as they get hotter (up to a point). So ensure your tyres are warm before attempting full racing speed.

Take everything out!
This should really go without saying but take out everything including the spare wheel, jack, luggage, coins, rubbish, children, seats etc This reduction in weight can make the difference between winning and losing. In addition, you don't want things rattling around while you're trying to concentrate, and in an accident anything that's left in the car is likely to hit you on the back of the head.

Remove obstructions from the radiator
If you're doing sustained laps, you'll need all of your car's cooling ability. If you have a turbo-charged engine, you'll probably have several radiators and intercoolers, all of which need a steady supply of air.

Some cars have number plates which partially obstruct the radiator's precious air flow, so remember take these off to give a bit of extra cooling.

Yourself:

Shoe choice
Shoe choice is very important when driving quickly. You need to have good pedal feel, and to be able to press the pedals confidently and accurately. A racing shoe / boot is the best bet, which have very thin soles with no overhang. Second choice would be a thin soled trainer.

Protective clothing
Even if it isn't specified for the event you're attending, it's a very good idea to wear a helmet and fire retardant clothing such as a racing suit. Crashes happen, and you'll need to be able to walk away smiling.

Mentality
It's very common to get nervous / excited before an event, but you can't let this go to your head. Ease into it, find the racing lines and braking points, and gradually build up speed. It's very common to go off on the first lap, so don't let it happen to you.

Enjoy!