The foot and ankle are a mechanically intricate complex of 26 bones, 33 joints, over 100 ligaments, and 19 muscles that must create integrated mobility and stability in order to support movement, balance, and standing activities. According to the American Academy of Orthopedic Surgeons, more than 43 million Americans, or one out of every six people, have problems with their feet, at health costs of more than $3.5 billion per year.
As a Physical Therapist that sees a high percentage of the foot and ankle injuries in our clinic, a recurring theme that I encounter is that of patients seeking help with selecting appropriate footwear. It is estimated that up to 87% of women and 60% of men are in ill fitting shoes. Considering that an average person will purchase anywhere from 450-600 pair of shoes over the course of their lifetime, and given that it is hard to get from point A to point B without putting your feet on the ground, purchasing appropriately fit shoes is a very important proposition.
Here is a very simple guideline for your next trip to the shoe store:
1) Fit Shoes at the End of the Day: Over the course of the day, our feet tend to swell and our forefoot will splay, altering the size and width of your feet.
2) Measure Both Feet: 75% of right handed people have a larger left foot. It is important to use a Brannock Device to measure both feet in a standing position. It is important to measure both length and width. When you step, your forefoot is designed to splay to provide high stability and adaptability to the surface you are standing on. This means that getting a shoe that is wide enough will ultimately allow for better foot mechanics during walking. 
3) Knee Alignment: With both shoes on, perform a half squat. The kneecap under normal circumstances should align with the space between the first and second toe. If the kneecap orients to the inside of the big toe, the shoe may be too soft or not supportive enough. If the kneecap orients to the outside of the second toe, then the shoe may be too stiff / rigid.
4) Pronation / Supination: With both shoes on you should be able to twist from side to side and have both of your feet pronate and supinate (rolling of the foot/ankle in both directions)
5) Break of the Shoe: When the front of the shoe is flexed the break should line up with the base of the toes or the metatarsals. This will encourage normal push off mechanics.
I also field a lot of questions from patients that have been diagnosed with over pronation, and are searching for motion control shoes for running. While it is important to note that not all people with flat feet are over pronators, there are a few key points worth mentioning.
1) Ensure the shoe has a straight last (the orientation of the shoe should be straight and not curve inwards towards the forefoot
2) There should be a strong heel counter that makes the shoe stiff and will help resist pronation forces.
3) The shoe should have a vertical orientation. If you hold a ruler or pen perpendicular to the floor, it should cut the shoe into two halves.
FINALLY a word of caution. The shock attenuating material that running shoes are made of tend to have a half life of 6-12 months. What does this mean? It means that after 6-12 months, the shoe loses approximately 50% of its shock absorbing potential, and this is very significant because the clock starts ticking from the time of manufacture, not from the time of first use. So, if you are a runner or a hiker, be careful when buying shoes from the bargain bin, as they are likely last year’s models.