04/24/2009 at 12:00 PM EDT
Beyoncé Knowles's curves are in all the right places – thanks to hours of dancing and online shopping!
"I saw this dress online. I'm always on Style.com
," she said of her sparkling Balmain dress with an endless black train on the red carpet at the Cinema Society and MCM-sponsored New York premiere of her film Obsessed
. When her stylist friend found it for her, "he was so excited, he said, 'Please fit, please fit.' "
And fit it did, but not without a lot of work on the part of Knowles. "I lost a lot of weight naturally, just from 12-hour rehearsals," she explained, attributing her recent weight loss to preparing to tour
. "That's a lot of dancing. I would never work out for 12 hours, but I'm able to move around and not think about it. It's part of my job."
Her other stay fit trick for fitting into those tiny tour costumes? "I've been trying to eat really clean," she told PEOPLE. "Fish and, in the morning, Special K."
Knowles, joined by Jay-Z in the theater and, surrounded by bodyguards at The Bowery Hotel dinner after, accented the look with shinning chrome nails and a unique bracelet. "It's black and white jade and white diamonds," said the designer Lorraine Schwartz at the screening. "It's incredible. It was designed with her in mind."
But, most importantly, is the star comfortable with the long train, metallic nails, and fierce four-and-a-half-inch heels? "I wouldn't say comfortable, but hopefully, I'll be happy when I see all the photographs and footage," Knowles said.