Socks:
1) Always try on boots with the same socks that you will be wearing on the slopes to get the best idea of how a boot will fit. Make sure that you only wear one pair and that they are sport specific. This will help to avoid getting blisters and better wick away sweat from the foot to the outside of the sock keeping your feet warmer.
Flex:
2) Always consider your type of riding and binding characteristics before trying on boots. Usually freeriders prefer a stiffer boot to provide quicker responsiveness for carving, while freestylers tend to like a softer boot for comfort and extra forgiveness when jumping. In addition, look at how stiff the binding backs are and get a boot that compliments them well in terms of your desired level of stiffness. If you do not already have bindings then get your boots first(always the best way to do it) as they can determine which binding will best fit the boot.
Liners:
3) With boots you either have a separate or built-in liner. A separate liner tends to pack out less and is easier to work on if you need customizing done because of pressure points on the foot. Built-in liners tend to have more cushioning, but pack out more also.Take notice to the type of materials used to make the liner, as this will play a role in just how much the liner will pack out. Generally, the more cushiony the liner is, the more likely it will pack out further thus making the boot roomier inside. A better liner will have good padding around the ankle area to prevent the possibility of heel lift when riding.
Proper Fit:
4) With the boot snuggly laced up, wait at least 5-8 minutes for the boot to pack out from pressure. To acheive this simply stand in place and drive your knees towards the gorund to simulate the riding experience. It is okay if the boot seems just a little too small. Remember that the tightest the boot will ever be is when you first put it on. The boot will only get bigger from that point on. After about 5-8 minutes, put the other boot on and you should immediately notice that the first boot is roomier than the second. This will simply further show you how much bigger the boots will become. Now with your feet about shoulder width apart and knees just slightly bent to the natural angle of the back of the boot, your toes should be barely grazing the fronts of the liners. This is considered to be the standing position in boots. Next, bend your knees further until you are in your snowboarding stance. Your toes will come back a little and should now be just slightly away from the liner fronts.
Heel Lift:
5) You may also want to look at the amount of heel lift you will have. The best way is to get secure in some compatible bindings and lean up against a wall to simulate riding. The secondary way is to have the fitter hold down your boot heels as you lean forward. A well fitting boot will improve your riding and help to make each day you ride more enjoyable.