The most painfully expensive part of the experience involved buying the crash helmet, which by law one has to wear in the UK.
My excuse is that I was in too much of a hurry to get a helmet of my own to use for my CBT (compulsory basic training, a one-day course which is required in most cases before you can ride anything over a 50cc in the UK). So I rushed out and got one the day before the CBT - instead of using a free-loan helmet from the training provider. A bad combination, fastidiousness and over-hastiness.
The result is an expensive helmet which gave me a headache after 15 mins, and so proved unusable. (Unsellable too, I suspect, owing to a very small nick in it caused by a friend taking it off a bit jerkily and touching part of a chair with it - an attempt to help stretch my helmet on a larger head, which sadly went awry.)
The duh points I should have known are these. Yes, I should have thought about them before I rashly rushed out to buy - but I didn't.
1. Is the helmet shaped like your head?
Believe it or not, people have differently shaped heads. And different helmets are differently shaped, even ones by the same manufacturer. Some will press on certain parts of your head (say if the top of your head is broad and the helmet is narrower there) - and that can hurt like hell after a bit.
Try lots and lots of different helmets until you find one that you feel very comfortable with, or at worst the least uncomfortable with (and ideally it shouldn't be uncomfortable at all, for safety too - you don't want to be focusing on the discomfort rather than the traffic), even after wearing it for a while. Which leads to point 2.
2. Take your time
You won't know how well or badly a helmet fits you until you've had it on for at least 15 minutes. 1 or 2 minutes is just not enough. Any retailer worth their salt will be willing to let you keep their demo helmets on for that long. Don't worry about looking silly standing around with different helmets for ages. Try as many different helmets as it takes, for as long as it takes. It's worth it.
Consider the retailer's advice as to whether a helmet is too big (and would slip and endanger you), but take it with a slight pinch of salt; if you think it's too big, don't necessarily take their word for it that it isn't. I had different sales representatives say different things about the same helmet and the second one was right, a helmet which the previous one said was fine was in fact too big for me. Taking your time is important for safety too - if your helmet is uncomfortable, you're going to be distracted while riding.
3. Ventilation, ventilation, ventilation!
If you have very thick hair, pick a helmet with more vents - ventilation makes a huge difference, or you risk headaches and heat exhaustion. I've rarely felt worse than with a too-small helmet which was so hot I could barely breathe, I felt literally sick.
Usually more expensive helmets will have more vents, including ones that you can open and close. Interestingly the helmet I eventually bought (the second helmet, I should say!) cost a bit less than the first one (the mistake), yet had more vents.
4. Glasses, alas
For those with poor eyesight, tough luck. You'll have to switch to contact lenses, or if you're one of the unlucky few who can't take contacts, get a pair of plastic framed glasses. Wire framed ones simply will not do. It's impossible to get them on over the helmet. Believe me, I tried (the retailer told me I wouldn't succeed, but I had to have a go for myself. They were right, of course).
And don't just get plastic frames, but (if you can manage a spare pair just for biking) get your friendly optician to straighten the arms so that they don't curl over your ears. This makes it much easier to get them on, in fact it's virtually impossible otherwise (if you're me, anyway). Plus you can pretend they're cool Oakleys, which also have the straight arms.
Finally, take them with you and try them on over your short-listed helmets. Some makes are more glasses-friendly than others.
5. Avoid false economy
As it's your head and your life at risk, if you can afford it, buy a more expensive rather than cheapo helmet. You do get what you pay for here - not just in terms of comfort (like vents, adjustable inner padding etc), but solidity and safety. Arai also offer to test the helmet for free if you've been in an accident, to check if it's still safe to use it. A full face one is also safer than a half-face, and I did manage to get over the slight claustrophobia.
So, that's what I learned. I certainly hope someone else will benefit from my £200 over mistake! And - I have one Shoei helmet for sale, black, only used once, with a small ding (barely noticeable dent in the paintwork). Going cheap. Anyone..?