A little discipline has gone a long way for the once beleaguered Britney Spears. The slimmed-down, argyle sweatervest-wrapped postbaby bod the sometime pop starlet happily unveiled Sunday while out shopping in Malibu, Calif., has earned her the cover of the latest Us Weekly, which trumpets, "How Brit Got Thin: 26 pounds in one month! No trainer, no pills."

Seems the comeback-geared Spears, 24 (yes, she's still only 24), has not only been working up a sweat twice a day since the arrival of Sutton Pierce on Sept. 12 but she's also apparently been saying no to yummy favorites such as whip cream-topped Frappuccinos and deep-fried fast-food delicacies.

"She is not treating herself with food and desserts anymore," shares an insider.

According to the mag, Britney packed on approximately 40 lbs. during her pregnancy, going from 120 to 160, but she has peeled off excess baggage through careful calorie-counting and exercise (it appears she didn't let a little thing like major surgery, specifically, a C-section, slow her down).

Her retail therapy last weekend was a "reward" for her successful weight-watching, with a staffer at boutique Planet Blue recounting to Us how excited Spears was over being "a whole new size -- an 8" (and somewhere, Rachel Zoe, the desiccated stylist to many a scrawny starlet, recoils in horror) and had to pick out "all new clothes."

Brit's reduction regimen reportedly includes running on a treadmill for an hour in the morning and lifting weights for 45 minutes at night. She's also eating six small meals a day and nixing white flour (so long, Wonder Bread, hello whole grains).

Although it's unclear whether Kevin Federline is picking up the poopie diaper slack for Sutton and Sean Preston while Britney feels the burn, the magazine says he's been very supportive of her diet dedication.

K. Fed's encouragement isn't surprising, given that he's tapped her to host the record release party for his long-in-coming debut disc "Playing With Fire" on Oct. 31 (good timing because those inexplicably compelled to buy it can do so while hidden beneath their Halloween costumes) and has been enthusiastically touting their collaboration on a ditty called "Crazy."

"After she has the baby and gets skinny, we're gonna hit 'em with this s**t," he explained to GQ in a prebaby interview.

But in between fatherhood and rap infamy, Federline, who has masterfully parlayed the public's profound scorn into a lucrative side career (please see his gut-punching on "CSI" and satisfying WWE smackdown), is still finding time for some fun.

In addition to his recent jaunt to Las Vegas, he also enjoyed a night out at Los Angeles hot spot Les Deux, reports People, which says he quaffed tequila shots and rebuffed a gaggle of seemingly misguided girls who "repeatedly" tried to have their picture taken with him.

Meanwhile, is Britney going to drop a baby name surprise in her expected People magazine exclusive? The rumor mill is abuzz with whispers that the Federlines didn't actually dub their new ankle-biter with the upscale moniker Sutton Pierce.

Kevin fueled the talk during a radio interview last week as he discussed the "crazy" stories that surround them, including a tale that baby No. 2 is really a girl. Said K. Fed, "Well, it's even crazier when people are telling me my child's name."

So is this just a little PR-generating bit of misdirection from Britney and Kevin, who have yet to confirm the tot's tag? Time will tell, although the Celebrity Baby Blog throws out the possibility that they may have christened him Jayden James, or J.J. for short.

And finally, don't expect Spears to be a guest judge on the upcoming season of "Project Runway." Tim Gunn, the show's Yoda-like and even-handed fashion guru, is uncharacteristically ruthless when asked to weigh in on the Britster's sense of style.

"Aside from looking as though she's just been dragged out from under a truck that tried to run her over, she always looks like she's in need of personal hygiene," Gunn blasts to Us. "The whole package to me is just repugnant. Ugh!"