Thanks to the ever-present paparazzi, we've all been watching Britney Spears shape up since leaving rehab a few weeks ago. "After all the negative stuff, she's getting back to her 'fighting weight,'" says one of the popstar's business associates. And a bystander was wowed to see Spears's new svelt bod as she shopped in Bel Air last week, saying, "She looked totally healthy and beautiful. and her legs are awesome." How's she doing it? The old fashioned way, according ...