A very good topic Andrew - there should be lots of useful info posted here.
FM2cWorth, the priorities are
1. SAFETY (big letters)
2. performance (little letters)
I will concentrate on safety here.
Safety is both car AND driver. The consequences of shunting a wall are just too horrible to think about, even if you walk away OK. I once saw a 308 go off backwards into a bunch of tyres (sounds soft enough, eh?), and afterwards they couldn't open the rear OR front bonnet/hood (the chassis was deformed at both ends)
Running gear will take a beating, esp tyres, wheel brgs, rims and suspension mounts. You will be cornering MUCH harder than on the road and hitting the ripple-strip when at max roll angle will put big loads on mounts, steering joints & hubs. Make sure all the suspension & steering is in top order with no slack or worn bushings, bearings, etc.
If the dampers/shocks are worn then you will get mighty "floaty" over any hills, and circuit designers just LOVE putting corners over the brow of a nice hill. :evil: Without tight suspension you will be hanging on by your toenails over the crest while also desperately trying to shed speed and turn-in. Ugh!
As Stradale says, brakes work hard. There is very little time for them to cool between beatings so put in fresh high BP fluid and heavy duty pads if available. Check disks for run-out as any warping will be a nightmare and make the car v. hard to control. Poor dampers will really hurt here also, as they won't control the weight transfer.
Don't forget the engine in the safety equation. If oil levels are low the cornering forces will starve the pump on a wet-sump model, and having too much oil will flood it up the side of the block into the crank's path. Either way you will have immediate problems just when you don't need any. Have safety gear for any emergency (fire, battery-disconnect, clothing, etc).
Finally, prepare yourself. There is always lots of "car-talk" about track days but the driver is more important. Don't drink or take drugs within cooee of the big day, try to get some good sleep, don't try to impress anybody "trackside". If you've been on the track before then running around the circuit in your mind, re-making mental notes of the key markers & positions will always be great preparation.
Good luck and enjoy - there ain't nothing to match the thrill. :green: