The fit of your ski boots is more important than just about any other gear decision you'll make. The hottest new skis or the lightest bindings can't make up for the lack of control you'll experience from an ill-fitting pair of boots.
The number 1 rule of trying on ski boots is "if they're comfortable right out of the box, they are too big!" Without major modification, this is the tightest they will ever be! After a couple days of skiing, they're going be loose and sloppy - a recipe for pain and discomfort.
Here is how to try on ski boots for the perfect fit:
Get yourself some thin ski socks, normally labeled with the words "thin" or "ultra-thin" on the packaging. Believe it or not, these will be warmer than the thick and fuzzy variety that will retain moisture. By starting with a thin sock, you'll give yourself room to move to a slightly thicker sock and maintain your tight fit as your boot liners break in.
Also, you'll want/need an aftermarket footbed. Brands like Superfeet and SOLE are readily available online and at most ski shops. A true, supportive footbed will prevent your foot from getting squished flat when you tighten your boots, resulting in a much more comfortable fit.
When trying on your prospective boots, with your thin socks and footbeds, a moderate amount of discomfort due to tightness is OK. Often this will dissipate in a few minutes, but if it doesn't, most boots have heat mold-able liners that can be formed to any foot shape. Remember, this is the tightest they will ever be!
It's also a good idea to do a shell fit - i.e. remove the liners and see how your feet fit inside the plastic boot shells. Slide your foot forward until your toes touch the front of the boot. Ideally, you'll have 10-20mm of room (1-2 finger widths) behind your heel. If there’s zero room, or you can fit your whole hand back there, consider trying a different size. Also try centering your foot in the boot. If your foot is touching, or really close to the plastic shell anywhere you may want to check out some other options before committing.
For backcountry skiing, I typically go with a slightly more relaxed boot fit than I do for skiing inbounds. Still, when I put my boots on at the trailhead, I like them to be just a hair too tight, which always ends up feeling perfect by the time I'm ready to ski downhill.
Once you commit and buy a pair of boots, mold the liners and get them on snow for a few days. It will take 2-4 days for boots to break-in before they might need some additional work. Part two of this series will get into more DIY techniques to dial in your boots for the perfect fit.