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After a year of bad losses - her boyfriend Tony Romo dumped her on the eve of her 29th birthday in July, then her beloved maltipoo Daisy was snatched by a coyote - Jessica Simpson has turned losing into a positive: she's back to pin-up girl form after reuniting with Mike Alexander, the trainer who got her into Daisy Dukes shape for 2005's The Dukes of Hazzard.

Before that, Simpson - who has a history of being more relaxed about her figure when she's in a relationship because, as she told WHO in 2008, "when I'm happy, my weight doesn't really [matter]" - had, says an insider familiar with the singer's workout plan, "really slacked off in her workouts."

What spurred her to get back in the gym? "She's doing a lot of photo shoots for her new shoe line," says the friend. "And she wanted to look rockin' hot for it." Which means she's running on the treadmill five times a week and working out with a combination of resistance bands and weights.

Alexander also had her overhaul her pantry, by grabbing a huge rubbish bag and tossing out chips, biscuits, frozen pizza and booze. "She's on a strictly dry diet right now," said a pal. "No alcohol at all." But the singer may sneak in one of her favorite skinny margaritas "every time she hits a goal weight," says the pal, who believes she wants to shed at least 7kg. "I think she's halfway there already."

Always refreshingly candid about her tendency to yoyo in weight, Simpson - who drew "fat" barbs in January when she displayed a fuller figure in her now-infamous mum jeans at a Florida chilli cook-off - has gone public about her new regimen, tweeting to her trainer on Nov. 11: "Why am I hurting from our workout earlier? I'm getting old! See you tomorrow with a pouty face and sore buns."

The hard work is paying off, says her sister Ashlee Simpson-Wentz, who also hired Alexander to whip her into shape after the birth of son Bronx a year ago: "My sister has an incredible body," the singer, 25, told the December issue of US Women's Health. "I feel sorry for anyone who would judge her because she's one sexy lady."

For the full story on how Jessica trimmed down, see this week's WHO Magazine, on sale Friday, Nov. 27.

See what else is in this week's WHO Magazine