Most importantly you should take your time! If possible, try footwear on in the afternoon because our feet are generally smaller in the morning and tend to swell towards the evening. The same thing also happens on walks and treks, so this should be taken into account when selecting your shoe size.
If you use orthopaedic shoe lifts or insoles, always take them with you when trying on footwear. It’s a good idea to take your favourite socks with you too. And ALWAYS try on both boots/shoes!
Sizes: You should be able to measure a finger’s width of space between your big toe and the tip of the shoe. This is vital so that they don’t collide with the front of the boot/shoe during descents. Check to see if the heel sits properly by trying on the unlaced show. If it’s loose at the sides, the heel section’s too wide.
Test your new pair of shoes/ boots for at least ten minutes, or even longer if you can. Walk around the shop, go upstairs and downstairs in them. Try to simulate ascents and descents in the mountains.