Shoe megastores.They sport dozens of brands, lots of bright lights, too many unprepared salespeople. Hundreds of brand names and competing technologies overwhelm and confuse even the most competent salesclerks, not to mention the innocent layperson, making sneaker selection a hit-or-miss prospect. Frustrated and uninformed, you empty your wallet for the first shoe that feels good, only to develop blisters or painful arches 20 miles down the road. Direct, informed questions can elicit useful information and help you select a good sneaker. More-interpretive queries-What's the best shoe for a heavy runner with ankle problems and a flat foot?-bring all sorts of crackpot answers. So, you ask, what does a guy need to know to negotiate the self-service world of sneaker supermarkets?