It's best to buy shoes later in the day when feet tend to swell. Do not rely on the size of your last pair of shoes since manufacturers sizing differs. Get both feet measured and buy for the larger foot. It is important to try on shoes with the socks or the hose that you expect to wear, as well as any orthotic devices that have been prescribed, since these can alter the fit and comfort of a shoe.
Some important things to look for involves rear foot support including deep seats and firm heel counters. A flared heel is best with higher heel shoes. Good cushioning, and arch support is important whether it's a sneaker, loafer, or oxford. Avoid flip-flops except for short periods of time at the pool, or the beach as they can exacerbate arch and heel pain.