I bought some boots about 3 weeks ago, I ended up with Nordica Speedmachine 10's for about £250.
I went to Ellis Brigham, and got a very good fitter (nice intelligent girl, very fit too

). They measure your feet in thin ski socks (don't wear thick ones), and that determines the boot shell size. Then you basically have to try on all the boots in that size, as they're all different. Some are narrower at the toes, some are wide at the toes and narrow on the calves. There's a lot of adjustment on the straps but some boots will feel great and some will feel horrible, even though they're the same shell size.
I wanted Salomon Impact 10's or Atomic M100s when I went in there, but the Atomics were too narrow in the calf, the Salomons felt ok but just a little odd. Head boots felt lovely and comfortable but were too flexible for me, so I went for the Nordicas in the end. Although I'm barely intermediate in terms of ski ability, I'm pretty heavy and have powerful legs so I had to go for a more expert level boot - purely as the lower end boots were just too flexible for me.
Once you choose the boots, they heat them up on a heater and then clamp them on your feet, the liners have conformable sections which mould slightly to fit better - but you need to have the basic boot fit right in the first place.
I went one stage further, I got custom footbeds made too (earlier this year for my snowboard boots). With these they take a mould of each foot and make a footbed that fits you perfectly. They do make a difference to the comfort, but are expensive. IIRC I paid £50 for mine, also from Ellis Brigham.
I know some people buy online to save money (I've done it), but please try not to, it's a false economy. You'd have to be incredibly lucky to get boots that fit properly. You can't guess based on your shoe size, my normal shoe size is UK 10.5 but my ski boots are 27.5cm shell which is about a UK 9 or 9.5 IIRC, my snowboard boots were a UK 11.