In the Store: Try on shoes at the end of the day, when your feet are slightly swollen, as they will be when you run. And always test fit with the socks you wear running.
If you're planning to run technical mountain trails, get a shoe with a wide toe box. The extra space offers some protection against unavoidable toe stubbing and provides room for the inevitable foot swelling that occurs over long distances at high altitudes.
In the Field: Hot spots during break-in? Use a small piece of duct tape on the inside of the shoe to reduce friction.
Need temporary winter traction? Twist eighth-inch sheet-metal screws into the outer edges of the outsole of each shoe. The hexagonal heads of the screws (#4 or #6 diameters work best) offer multidirectional grip on ice, hardpacked snow, and frozen dirt.
At Home: Store your shoes indoors at room temperature. Running shoes left on the cold porch in winter will be rigid when you put them on, increasing the chance of blisters.