This article is from the Hiking Boots FAQ, by Chris Gilbert firstname.lastname@example.org with numerous contributions by others.
A good shop will let you try the boots on in the shop and will
invariably provide you with some walking socks to use while doing
so. They will let you pay for the boots and take them home so that
you can wear them around the house for a couple of days. If they
turn out to be really uncomfortable then, as long as they have not
been taken outside the house or damaged in any way the shop
should either allow you to exchange them or give you your money
or a credit note back. Don't take our word for it though, check with
the shop before you buy.