With all the fresh food and outdoorsy activities, you’d assume that summer must be very ab friendly. “But while people typically associate the holiday season with weight gain, I’m now seeing women putting onmore pounds during the warm weather,” says Keri Gans, R.D.N., the author of The Small Change Diet. The holidays are a month of special-occasion eating and drinking, while summer is three months of parties, barbecues, weddings, vacations, and weekends spent lounging instead of lunging. On top of that, there’s the burnout factor. After monthsof being disciplined with diet and exercise, most want to let loose in summer. “Basically, September is the new January—the month people try to take off the weight they put on,” Gans says. Not necessary, though—you can hold on to the results you’ve worked…