Britney Spears Getting Back to 'Fighting Weight'

04/18/2007 at 09:15 PM EDT

Since leaving rehab in March, Britney Spears seems to have dropped her hard-partying ways – and a few extra pounds.

"After all the negative stuff, she's getting back to her 'fighting weight,' " one Spears business associate tells PEOPLE.

That much was evident April 12, when the singer, 25, stopped by Bel Air boutique Sydney Michelle to stock up on supershort dresses. "She looked totally healthy and beautiful," says an onlooker. "And her legs are awesome."

What’s the secret to her seemingly sudden slim-down? "She's working out, eating right and enjoying life," says the singer's rep. (Some published reports also claim Spears underwent a nonsurgical liposuction procedure called LipoDissolve during a recent trip to Las Vegas.)

The mother of two – sons Sean Preston, 19 months, and Jayden James, 7 months – has also been sweating off the pounds with frequent trips to North Hollywood's Millennium Dance Complex.

"She's got an amazing metabolism," owner Robert Baker tells PEOPLE. "When she puts her mind to it, she can transform pretty easily."

Now that rehab and divorce talks with Kevin Federline are behind her, Spears's next priority is getting her career back on track. "We're working on the album," says producer (and former flame) J.R. Rotem.

So will this healthy new outlook finally get Britney back in the zone? "I hear she's taking care of herself," says longtime pal Lance Bass. "She's definitely getting her life back in order."

• Reporting by VANESSA DIAZ, MARK GRAY, AMY ELISA KEITH, LYCIA NAFF and NICHOLAS WHITE