TRACK DAY VIRGINS GUIDEOK, some valid points have been raised, and some obvious ones missed.
The circuit. Castle Coombe is a good "drivers" circuit, but in some places has little or no run-off. You MUST listen to the pre-event breifing, to listen for any specific hazards - ask the organisers if they found anything notable on their morning track inspection - if they havn't done one, I'd be worried! Secondly, you MUST use the sighting lap to assess for yourself which corners have particularly dodgy looking run-off areas.
The organisers. Who are they? There are some excellent orgainisers who have been doing track days for years, and there are some truely shocking organisers, whom I wouldn't trust to organise a Teletubbies tea party.
The format. Is it grouped by ability or not? IE, is it split into "novice", "intermediate" and "advanced" - or is it a general free-for-all. Secondly, timings. Is it split into timed sessions, ie, the novices go out for 20minutes, then the inters, and then the advanced - or is it an "open pit lane" format? If you have never done a track day before, do NOT start with an open pit lane event.
The "attendees". Is it open to only members of a certain group of drivers, such as members of GolfGTI forum or Volkswagen Driver Mag subscribers - or is it a general free for all? If it is a free for all, try to get an advanced list of cars already signed up - if it is already bursting with Saxos, Novas, Corsas, Fiestas and the like - then don't even bother going. If there is a list of more "mature" cars, such as Porkers, Fezzers, Astons, BMWs, Audis, Mercs and the like, then hopefully, the drivers shouldn't be trying to prove anything to anyone else!
Your car. Obvious really - make sure you have a full tank of fuel (you know only V-Power or Tesco 99 will do

) when you drive through the circuit gates, AND, make sure you know where to get some more during the lunchtime break! Under-bonnet levels - make sure your oil is to the max on the dipstick (not even halfway down will do), the brake fluid, coolant, and screen wash are all topped up to the max. Take a spare litre bottle of oil, and check the level during the lunchtime break. Inside the car, clear out any unnecessary luggage from the boot, back seats, glove box, door bins etc. Remove any furry dice/magic trees from the mirror etc. Fold the rear seats forward, remove the rear parcel shelf. Take with you a 2" wide roll of duct tape, and a 3/4" wide roll of electrical insulation tape. Use the duct tape to fully cover your number plates, and the insulation tape to criss-cross your headlamps and front fogs (you can do your rear lamps, and your exterior mirrors too, but I personally find tape on the door mirrors a distraction). Fix the towing eye to the front (and make sure you tighten it with the wheel brace). Brake pads, make sure these have plenty of meat left on them - if not, consider getting some Mintex 1144 fast road pads. Tyres, make sure they have at least 4mm tread, and pump them UP to the MAX pressures. Under NO circumstances, never reduce the pressures "for track days", as you will simply put extremely excessive loads on the outer shoulders and sidewalls.
The driver. Get up EARLY. Allow yourself plenty of time to get to the circuit (if you miss the briefing, you won't be allowed on track!). Try to have a light breakfast before you get there, not a greasy fry-up. Don't drink too much coffee. Don't wear tight clothing, and don't wear jeans. Light-weight non-restrictive clothing, even jogging bottoms and fleeces are better than any kind of denim or heavy cotton clothes. Helmet - you will need one. Try to dig out your old helmet from your "moped days", rather than borrow/hire the ones available at the circuit - as they are likely to be truely manky. If you have to hire a circuit helmet, buy a thin silk or similar balaclava to use under the helmet. Footwear, use the lightest, softest footwear you can find - trainers are OK, rigger boots are not!
Contingency plan. Sadly, bad $h!t does sometimes happen. Consider track day insurance, and also consider recovery home. Hopefully, you have a truely safe day, keeping away from the armco, and clear of the kitty litter - however, you may well find that your tyres have worn through to the canvass, or you may have cooked your brakes!
to be continued . . .