Buying ski boots is one of the most important decisions that you can make in your skiing career and it is not too overdone to say that a bad decision here has hampered many people's enjoyment of their subsequent ski holidays.
Ski boots should never be bought without the chance to try them out on the snow. No matter how long you spend in them in the shop they will never feel the same as they do when you are skiing.
That means that you should buy them on holiday in the snow, at an artificial slope near your home, or buy the exact same model that you have tested on a previous ski holiday.
There is no longer any need to wear thick socks or two pairs of socks - ski boots these days should be closefitting and warm enough to allow you the better "feel" that normal socks will give you.
Front- or rear-entry? The old debate. Rear-entry for ease of use, front-entry for closer control over the skis is the rule of thumb, but here it helps if you are honest with yourself. If you are not looking for a stiffer performance pair of boots in the first place, you are not going to get the benefit from a front-entry pair as opposed to a rear entry pair.
How should they fit? They should be snug with no pressure points on the toes or at the widest part of your foot. Your heel should not move markedly up and down when they are closed. But you shouldn't be able to do more than wiggle your toes around. Don't go for a more sloppy pair because they feel more comfortable - you will end up sacrificing control over your skis in the process.
Lastly, consider custom sole inserts or the special foam-injected boots which mould themselves to your feet. The extra price is, say many happy users, more than made up for by increased control and comfort.