I was bored, so knocked this up for anyone who hasn’t done (but is thinking of) doing a trackday. This covers everything I can think of off-hand, but is by no means exhaustive. Please feel free to add to them. These are based on my experiences (11 trackdays thus far), so not to be taken as gospel, just advice!
Do you need a full licence or is 33bhp okay?
This varies from trackday company to trackday company. Some specify yes and others not. I’ve been on trackdays where I haven’t even been asked to produce my licence at all. Others have checked it very thoroughly. Don’t assume you will be okay; check, because you won’t get your money back if you get it wrong.
What do I need on the day?
A bike (pretty obvious this one), a helmet (which must bear the British Standards kitemark), gloves, boots, and one or two-piece (zipped) leathers. Hire bikes can be had from a few places like Lady Snoots, or Focused Events. Oh, and you’ll need a licence too.
If you’re riding to the track and you know someone taking a car then a tool kit is also a good idea, and spares like levers and such if you have them.
What can I expect on the day?
Expect sign-on to take place from around 7:30am. This will involve you showing that you’re there, signing disclaimers and liability forms and collecting your wrist band (which gets checked before heading onto the track). Generally speaking there will then be a briefing at around 8:30ish. Here, someone from the organiser will talk about the track, about track etiquette, the timetable for the day etc. He will also run through the different flags and what they mean. The briefing is compulsary, no matter if you are Valentino Rossi.
Normally you have three groups; novice, intermediate and fast. If you’re making notes on this then it’ll be your first trackday and you should really have booked into novice. Generally speaking you’ll be last out on track. Don’t worry, if you do turn out to be a track-god you will get moved into a faster group. Likewise if you think that lapping the local roundabout makes you Kevin Schwantz, but in the real world you hold up five bikes on every corner in the fast group. You’ll soon get moved. The groups are annouced normally over a PA system and will go out for 15-20 minute sessions. This means that you’ll get 30-40 minutes break in between your sessions out on track. Save for an hours break for lunch this will continue until about 5pm (or failing light, depending on time of year). Expect to get somewhere in the region of five sessions during a day. It may not sound a lot, but you will be tired when you leave. On a day last year at Donington I managed 160 miles on track, although that was unusual.
Normally there will be instructors on hand for free advice. If you’re quick enough (I mean running, not on the track) you can grab one for some free tuition. He’ll follow you round, watch your lines and offer advice where you can improve. You can then follow him and learn the lines. There will normally be a tyre guy on site, offering a tyre fitting service and sometimes a suspension bloke offering customised adjustments for you. Finally (aside from the medics and marshalls) there will be a photographer so that when you get your kneedown you can email the picture to everyone you’ve ever met and post it with infinite smugness on every forum you can find. 
My biggest advice on the day? Relax and enjoy yourself. You won’t break any lap records and you won’t be the fastest person there, so don’t try. You’ll either end up frustrated or in A&E. If someone passes you, let them go, they’re faster. If you give chase you run the severe risk of outstretching your ability and crashing.
Will there be fuel available onsite?
Tricky one, and one that needs to be checked. Some tracks have onsite petrol stations (which are not always open) and some don’t. Someone will be able to direct you to the nearest petrol station should you need some during the day and there isn’t any onsite. Bear in mind it can be a pain in the arse to reattach mirrors, numberplates etc. If you can, fill up as close to the track as possible in the morning. The other option (if you have a car-driving friend going) is to fill a jerry can.
Will there be refreshments?
Undoubtably. The scale will vary from a burger van to a full canteen setup, but there will be something. Don’t eat so much that you’re lethargic and bloated (especially at lunch) but do keep yourself hydrated.
Do I need to “track-prep” my bike?
You don’t need to do anything. However, I would advise that firstly you remove or tape up your mirrors; you don’t need them. There is absolutely no need for you to be looking behind you. If someone wants to overtake it is their responsibility to do so safely and without impeding, or unsettling you - this is not racing. Looking behind you will only draw your attention from what is in front of you, which cannot be a good thing. Removing the mirrors (rather than taping them) means one less thing to replace should you stack it! 
Some people remove or tape up their numberplates. Why? Well, maybe they want to be in a position to claim from insurance should they crash and don’t need evidence popping up at a later date showing that they actually crashed on a trackday and therefore the claim is fraudulent. There are also rumours that people from insurance companies visit trackdays for this very reason and make notes on bikes. It could be that you could have an entirely legitimate claim turned down if the company saw you on a trackday a couple of days before your real accident. It’s unlikely, but why risk it?
The other favourite it to tape up lights and indicators. This stops glass/plastic showering the track should you come off. The real reason, however, is that it makes you look like a racer!
Check, and drop the tyre pressures. Normally there will be a tyre van at the track and you can generally ask them (or an instructor) for some free advice. Expect to drop the tyre pressures to about 31 psi when cold (it will vary between bikes), the reason being that the tyres will be working a lot harder and getting a lot warmer than they ever do on the road. As they get warm the pressure will increase, and you don’t want over inflated tyres. Someone will have a pressure gauge if you don’t. Don’t be afraid to ask around.
It goes without saying that you should have checked your oil levels, tyre wear, chain etc before the day.
What happens if it rains?
You’ll get wet. It takes a lot for the entire day to be cancelled (I’ve only known it when there was snow and ice on the track) so the chances are you’ll be allowed on track. Whether you go out is up to you, many people don’t. If it gets really bad during the day they might stop letting people out until it clears (this will vary greatly between organisers and tracks). If the day goes ahead that’s the end of it; no refund, no rebooking.
Will I get laughed at for being slow?
There will always be someone slower than you. If you are that guy, I’m sorry, what can I say? I have seen people at trackdays on bikes never designed to go anywhere near a track and riding with three inch chicken strips. Are they slow? Yes. Do they hold people up? Of course not. It’s a race track, there is so much room in most places you won’t believe. Donington Park is 10m wide. If someone can’t get past you it’s not your problem.
Finally, some useful Links
I won’t list every trackday company, simply using the two links below will show you most days on offer currently and their details:
Circuit information:
http://www.racingcircuits.net
