The vast majority of recreational skiers buy boots that are too big. They feel great in the shop, but once you've skied for a few days, the liner (the foam layer between your foot and the plastic shell) becomes compressed, or "packs out." Suddenly your boots feel sloppy: Your foot may twist laterally within the boot (A), or your boots may feel too long and your heel might rise because it's no longer snug in the heel cup (B).
Buy boots that feel snug at first, perhaps a size smaller than your street shoes. (A good fitter can always make a boot bigger, but never smaller.) Your toes should almost touch the front, your heel should feel locked down and your entire foot should feel enveloped, like your hand in a firm handshake.
Wear one pair of ski socks when you try on boots. (The only things that belong in your boot are your foot and one sock—no cuffs or stirrups.) Avoid preconceived notions about brands. Favor the models that best match your foot.
Beginners should look for "soft comfort." A boot with a soft flex will absorb shock and gross body movements. Find boots that are easy to get on and off. Intermediates want boots that are soft fore and aft, but stiffer laterally for better energy transmission. Look for boots with medium volume and lots of adjustments.
Advanced skiers and experts should seek "harder comfort"—stiffer shells that cancommunicate even subtle movements from your feet. Freeride skiers may look for boots that offer shock absorption. Expert boots generally have the lowest interior volume. Don't confuse a "race fit" with a boot that's too tight. Although the shell closely matches the shape of the skier's foot in a race fit, the foot and ankle still have a little room to play.