I know it's tempting to hold onto your trusty running or walking shoes until they begin to (literally) fall apart, but you're just begging for an injury if you do that. As time goes by and the miles pile on, running shoes begin to lose both their stability and shock-absorption power; for walking shoes, the big problem for most people is that the outer heel begins to wear, which can throw off your gait.
So the solution, of course, is to replace your kicks before any of that happens. But at what point? Here's a great rule of thumb (or foot), courtesy of Stephen M. Pribut, a fellow at (and past president of) the American Academy of Podiatric Sports Medicine, who's been practicing podiatry for 33 years (read: He knows his stuff!).
Running shoes: Replace them every 350 to 450 miles—which, for those who don't feel like counting every step, equals to about six to nine months for the average runner.
Walking shoes: Assuming you walk somewhere between 1 and 5 miles a day, the magic time frame is once again six to nine months.
So now, of course, the trick is to remember to actually do the replacing. Try this: Figure out the date on which you'll need to make the switch, then either enter it as a to-do in your calendar or write it on the inside tongue of one sneaker. Voilà!
One more hint: Next time you find a pair of running or walking shoes you think you can't live without, buy two (or three or four) of them. That way, when it comes time to do the Big Change, you'll have another great pair ready to hit the road.