By staying indoors as much as possible. I think the weather is one of the lesser factors (albeit still a factor) for obesity in the South. For a decent chunk of the year it's just downright unpleasant to be outdoors and so folks just sit around inside eating or whatnot. When I lived in Colorado, the skinniest state I believe, people were always outside doing something during the spring/summer because the air was so crisp and the temperature so mild.