The start point in fitting the boot is to ensure that you have the right size and remember that it is possible to customise the fit of a boot that is slightly too large but it is virtually impossible to do anything about a boot that is too small. If in doubt go for a size that is slightly large rather than one that is too small. Bear in mind that the critical size element is the length. Boots will often give or can be modified in width terms but it is virtually impossible to modify the length of a boot.
When trying the boot on make sure you do so with your usual sock combination. You should also bear in mind the end use of the boot. For example if you are going to use your boots when you are likely to be carrying a heavy rucksack then this will alter the loading and shape of your foot. You should therefore try wearing the boots with a loaded sac on. Also boots used for the mountains will be used in an uphill and downhill mode. You should therefore simulate this when trying the boots on.
An increasing number of specialist shops are now providing 'incline boards' for you to test boots on. The key points to bear in mind are that when testing the boots in a down hill mode, properly laced up, your toes should not be pushing against the front of the boots. Equally, there should not be as much of a finger width at the heel otherwise your boot volume is too large for you.