Jennifer Aniston and Mandy Ingber
keeps her friends close and her yoga instructor, Mandy Ingber
, even closer.
But Aniston, 43, says a lot has changed since the fitness friends first met 22 years ago.
"We were a lot rounder," Aniston said with a laugh at the Self magazine-sponsored celebration
of Ingber's new book, Yogalosophy: 28 Days To the Ultimate Mind-Body Makeover
(available May 6) at West Hollywood's Soho House. "We ate terribly. We did that, and then we sort of grew up a little bit more and we got jobs."
Ingber – who has shared her health tips in a weekly blog for PEOPLE.com – notes that she's witnessed Aniston's transformation over the duration of their friendship.
"It's hard to say is that yoga or is it as we get older we grow into ourselves, but I've seen her become so empowered, so grounded in herself, so comfortable in her own skin," Ingber said. "I've definitely watched that process happen and unfold, and I'm grateful that I've gotten to be the observer of that."
But Aniston isn't the only famous figure Ingber is responsible for: she also works with Kate Beckinsale
, Brooke Shields
and Helen Hunt, and the actress credits Ingber's approach to yoga with helping her sculpt her enviable body.
"That was always my 'ugh', my legs," Aniston explained. "That's why I credit her for [them] – with yoga."
Although she remains dedicated to her healthy lifestyle, the bride-to-be
admits to having a cheat day every now and then.
"Cheat days for me are usually ... it's either Mexican food ... maybe Italian pasta, but I think you always have to do it in moderation."
Join Mandy for 28 days of healthy body and mind tips, kicking off May 6 on PEOPLE.com. You can also visit Mandy on her website MandyIngber.com and follow her on Twitter @msmandyingber and Facebook.com/yogalosophy.