Helena Christensen bares it at 40 in Elle UK. Photo: Jan Welters/British Elle

Although Helena Christensen has been in the modeling industry for over 20 years, she shows no sign of slowing down.

Besides posing (often nude) for countless magazines, the 41-year-old mother of one just inked deals to be the face of Reebok's new sneaker (see the sexy ads here) and Bali's Comfort-U bras.

In an effort to support the Breast Cancer Research Foundation, Christensen is also helping Bali get the word out on its new Web campaign to raise $25,000 for the cause.

Here, Christensen talks to StyleList about her work with Bali, why posing nude feels "natural," and her secret to longevity in the fickle modeling world.

StyleList: Tell us about your work with Bali and the Breast Cancer Research Foundation.
Helena Christensen:
I shot the Bali Comfort-U campaign. We are encouraging women to go on balicompany.com, and for every click on the U symbol, Bali will donate $5 to the Breast Cancer Research Foundation until we reach $25,000. It's a very easy way for anyone to participate.

Do you have a personal relationship with breast cancer?
Thankfully, no. It hasn't affected anyone close to me, but I know friends of friends [who have been]. One in three women gets breast cancer, and men, too, which I had no idea about. I have so many wonderful women in my life, so it's always been a charity I've wanted to work with.

SL: You're naked except for sneakers in the new Reebok EasyTone campaign, and you've posed nude before this. Do you ever get nervous?
It's honestly such a strange job without even having to take your clothes off. You're basically the product. Twenty years later, I'm quite at ease because I've done it for so long. It's a very natural thing for me, and I probably look at it a little differently than other people.

For Reebok, I was shot by Steven Klein, and when you work with someone like that, whom you admire and know is a real artist, I will do anything. I look at it like I am a tool for his art. I don't see it as "I'm nude in this picture, showing off my body," I see it as how the composition is, how the light is, how the lines can form organic shapes with the body and background. I don't think, "Oh my God, I'm going to be naked." That might be in the back of my mind, but I look at it as collaborating, and I'm always up for a new challenge.

SL: You have such an amazing body. What's your workout routine?
I only got into working out about three years ago. It never really crossed my mind. I started by going into the most hardcore exercise there is, which is old-fashioned boxing. I needed a sport that constantly gave me new challenges, both physically and mentally. It's the perfect way for me to work out, and not just to be slim, but to feel fit and strong. I also run a bit and I just started to play tennis. But I don't go crazy when I run. After 20 minutes, I'll come home and feel like I'm going to collapse.

SL: I'm sure you listen to music when you run. What's the most embarrassing song on your iPod?
Some music is looked upon like it's cheesy or uncool, but not to me! I have a lot of Britney Spears songs I really like. "Toxic" is a great song. My son is kind of surpassing me now with knowing about music. He's very into Linkin Park and Owl City. I'm teaching him about The Cure and Depeche Mode, and he's teaching me. It's good to have a 10-year-old for that.

SL: Can you share some of your favorite beauty secrets or products?
It'd be hard to mention one beauty item. If you saw my bathroom, you can hardly get in there. I love skin products and tonics and serums. I always say, "Don't get anymore, just use what you have." But then I go into Sephora and Bigelow, and I'm like, oh man, this is impossible. Lately, I've been using Bliss's mint soap bar. It's got little, tiny grains so it stimulates you and smells really great. I love when you use a product and it has instant gratification.

SL: You're still in demand. What's your secret to longevity in the industry?
Knock on wood! I look at pictures of myself when I started, and I was very fortunate to have worked with great photographers right from the beginning. I feel that was a really lucky break. I don't know how they saw through my baby-fat face and perm!

When you work with certain people constantly, there's a friendship that forms and you start trusting each other. You feel like you are growing and accomplishing something together. I think when I started, it was about working with a group of people again and again, and I formed those relationships. For a lot of girls, it's a constant fight to find a job. It's not easy, and I'm very lucky that I am still working.

And we're equally lucky to observe the results.

In related news, check out other stars who have stripped for charity.