Measure your foot frequently. "It's a myth that foot size doesn't change in adults," says Steven Raiken, MD. "It does change as we get older, so have your feet measured twice a year. Sizes also vary between brands, so go by what fits, not by what size the shoe is." Raiken is director of the foot and ankle service at the Rothman Institute at Thomas Jefferson University Hospital in Philadelphia.
Shop toward the end of the day. Feet swell over the course of the day; they also expand while you run or walk, so shoes should fit your feet when they're at their largest.
Bring your own socks -- the ones you wear while running or walking. If you wear orthotics, bring those, too. Shoes need to fit with the orthotic inside.
Don't believe in breaking in. Running and walking shoes should feel comfortable right away, Raiken tells WebMD. Walk or run around the store a bit to make sure they feel good in action.
Use the rule of thumb. There should be about 3/8-1/2 inch between the front of your big toe and the end of the shoe -- about a thumb's width. The heel should fit relatively tightly; your heel should not slip out when you walk. The upper part of the shoe -- which goes over the top of your foot -- should be snug and secure, and not too tight anywhere. The American Academy of Orthopaedic Surgeons writes that when fitting in to an athletic shoe you should be able to freely wiggly all of your toes when the shoe is on.
Understand the bells and whistles. Some models of running shoes look better suited to a space mission than a run in the park, but some of those groovy-looking features actually serve a purpose. Clear inserts, filled with gel, Freon, or air, provide extra shock absorption, as do those springy-looking things. These features are especially good for people who tend to get heel pain, says Raiken, and not so good for people whose ankles twist easily, as shoes with extra cushioning tend to provide less traction.