If word got out that an ultra-famous celebrity used peanut butter on her face to prevent wrinkles, chances are, there would be a run on peanut butter at the grocery store. That's because for better or worse, we care which products celebrities use. And when they tell us, we want to use them, too.

How is it that celebrities always seem to be on the cutting edge when it comes to the latest fashion and beauty trends? We asked Rachel Weingarten, a New York City-based celebrity makeup artist, to let us in on some star secrets.

iParenting: Do you think makeup trends follow celebrity style or vice versa?

RW: I think it's a collaborative process – what do the fans want to see on their favorite star? Which makeup looks do they love so much that they'll start to copy them? If fans and the press love a certain look, you can be sure that a celebrity will wear it; thus, it will get photographed even more, and then more and more people will copy that particular look. Suddenly, a makeup trend is born!

I also think that there's some sort of cycle that occurs. For instance, I like to create several variations on a new makeup look for a celebrity, be it for a runway show or concert tour or even an awards ceremony. We'll have a few application trial runs where I'll apply my client's makeup in a variation of the original design, and my client will go to an event and see how people react – whether they love it, hate it or don't even notice it. What tends to happen is that fans, newspapers and magazines will pick up on one particular look, and then women will copy that look. All of a sudden, it's not described as that celebrity's "defining look." So then we'll go back and try to pinpoint the exact quality that people responded to and make that the focal point of her new look. It's cyclical.

iParenting: How do celebrities manage to stay one stylish step ahead of everyone else?

RW: A good example is the "Madonna phenomenon." Madonna follows a different set of rules than anyone else. She's kept our interest – for better or worse – for the last 15 or so years by constantly changing her look. Even more than being a trendsetter, Madonna's real skill is for being a trend-spotter. Madonna will incorporate the trends of the "fabulous" people and club kids in big cities like New York or London and incorporate these trends into her own style. Suddenly when Madonna is spotted in her latest incarnation with makeup to match, kids from small towns everywhere are in awe of her innovativeness and jump on the bandwagon to copy her. Incidentally, many fashion designers do what Madonna did with her Material Girl video, in which she copied the look of Marilyn Monroe in the film Gentleman Prefer Blondes. Designer Isaac Mizrahi is famous for watching old movies and updating the designs that he sees there for modern fashionistas, most recently in his new collection of shoes.

iParenting: As a makeup artist, what have you found most challenging about working with celebrities?

RW: I think when you work with a celebrity there are two real challenges: The first is that you are so used to seeing her face everywhere, you're almost as familiar with that face as your own. As a result, it can be a challenge to create a new look for them. For example, if they're known for their famously smoky eyes and ruby lips (think Catherine Zeta Jones), do you really want to mess with her beauty recipe for success? Although I admit that I am dying to give her an earthier look with caramel-colored lips, clay-colored eyeshadow – more of an early Sophia Loren look.

Many celebrities tend to see themselves in the light that their fans do, and it can be really hard for them to live up to that "larger than life" issue. Which brings me to the second challenge: Creating a fresh look for a celebrity client while preserving her signature look and keeping her fan base happy. It's a tough balance to strike sometimes – keeping the celebrity and her fans happy with her look while making her look dazzling enough to keep everyone talking.

iParenting: Have you seen that the kind of products the celebrities choose or prefer reflect their personalities? How?

RW: As far as actual products, again I'd think that this process is cyclical. For instance, you'll have a celebrity who becomes addicted to a certain product or brand, and then the brand will pay homage to her by creating a new product inspired by that celebrity. Lately the trend has been lipsticks named for celebrities. Makeup artist Carol Shaw has done that with her makeup line, LORAC. She has named nearly every lipstick in the collection after the celebrity that inspired the look.