Shop at the end of the day. Your feet can swell as the day goes on. Trying on shoes in the evening will ensure that you find something that fits you at any time of the day.
Don't be too flexible. To make sure a shoe provides enough support, try to bend it in half, bringing the toe to the heel. If it folds, keep shopping.
Find space in the right place. Look for a shoe that's snug in the heel but gives your toes enough room to wiggle. There should be about a thumb's width of space in the front of the shoe.
Break them in. Let your feet get used to your new shoes by wearing them on a couple of short walks close to home before setting out on a longer trek.