Credit: PacificCoastNews.com

Jessica Simpson is slimming down for the gown!

To prep for her upcoming nuptials to fiance Eric Johnson, the 30-year-old singer has enlisted the help of celebrity trainer Tracy Anderson, who's customized a routine that includes dance aerobics, ab work and strength training.

"She wants to look healthy," Anderson tells the new issue of Us Weekly (on stands now).

After a few weeks of 75-minute, four-day-a-week workouts, "she's seeing results," says a pal. "She really loves it. She thinks it's so much fun."

Anderson, who's also worked with Jennifer Aniston and Nicole Richie, adds that the $1 billion fashion mogul "is in the mind-set that this is a big, forever lifestyle shift."

For more details on how the margarita- and guacamole-loving star is revamping her diet -- and how Anderson plans to amp up Simpson's workouts -- pick up the new issue of Us Weekly, on stands now!