To paraphrase Johnnie Cochran, "If the boot don't fit, you might as well quit." Skiing, that is. Your ski boot is the single most crucial component of your gear system. It is steering linkage and suspension rolled into one. If your boot is too big, too small or too stiff, the connection to your ski will be compromised.
Unfortunately, buying ski boots is a process that can take hours, if not days. You've probably spent an hour selecting a pair of cross-trainers¿and they're made from soft materials. A rigid plastic boot shell will never conform to your foot's subtle nuances, no matter how long you wear it, so it's worth investing the time to find a good fit.
Most shops carry only four brands, so visit more than one if possible. City shops are least busy midweek and at odd hours, especially before ski season. Resort shops are jammed at the beginning and end of the day, so be prepared to trade skiing time for personal attention and go when everyone else is on the hill.
In the shop, try all brands, in different models and sizes. When you find a "winner," make sure it fits both feet well, then keep one on to compare to other models/brands.