Not those awful high-street chains staffed by uninterested couch potatoes, but the small specialist ones. They ask relevant questions about how you use the shoes and what sort of wear your old shoes are showing. They take a little video of your feet on a treadmill and make you feel good about how stable your ankles are (I suspect the roll of fat stabilises them nicely 
). At the end of it all you walk out with exactly what you came in for, a perfectly fitting pair of trainers.
If only all shopping was as straight-forward and satisfying. 