Royal Wedding: The Kate Effect

LONDON, March 21, 2011— -- They call it "the Kate Effect" and it's Kate Middleton's influence on the world of fashion.

The blue Issa dress she wore for her engagement announcement sold out of London stores within 24 hours.

"The three things that worked so perfectly with the dress was the fact that the color was a perfect regal blue," style expert Suze Yalof Schwartz told ABC News.

"She showed cleavage but it wasn't too much and the nip at the waist shows off the figure but it skims the body, so a lot of women can actually see themselves wearing it. It's a very flattering dress."

ABC News met with Daniella Helayel, the woman who designed the famous dress. So what's the secret?

"Blue is a quality we equate with Easter," Helayel said. "Every season, I go through shades of blue."

As for the cut, "I've been making that shape since '04," the designer said.

She didn't even know Middleton had chosen to wear one of her numbers: "I had no idea, that was a surprise," she said.

"It was amazing. Me and everybody else; all the girls in my studio."

Helayel dresses princesses-to-be but she also has tips for the rest of us. "If you find a shape that suits you, have that made in every single color. The body doesn't change. In the end, it's about what makes you feel good."

Meanwhile, on royal watch, Prince William is coming to the end of his whirlwind royal tour of New Zealand and Australia, and more wedding details are emerging.

"Carole Middleton and Pippa -- Pippa, of course, is chief bridesmaid in charge of all of the hen party arrangements -- are planning a "Dirty Dancing" theme at the Buckleberry family home because Kate Middleton's a huge fan," royal expert Katie Nicholl said of the bride-to-be's mother and sister.