Ever thought Ashlee Simpson's "Pieces of Me," Hilary Duff's "Come Clean" and Lindsay Lohan's "Over" sounded like they were written by the same person?

Well, Kara DioGuardi can explain why. She wrote (or co-wrote) them -- as well as Britney Spears' "Brave New Girl," Kelly Osbourne's "Shut Up" and Gwen Stefani's "Rich Girl."

Although she never intended for it to happen, DioGuardi has fast become the go-to songwriter for the princesses of pop.

Other clients include Jessica Simpson (she also wrote Ashlee's "Shadow," about Jessica), Kelly Clarkson, Celine Dion, Raven-Symone, Diana DeGarmo, Haylie Duff and Thalia. And to be fair, she's also penned tunes for a few pop princes, including Ricky Martin and Clay Aiken (his hit "The Way").

Simply put, DioGuardi is owning the pop charts, and perhaps even more impressive than that, she's doing it under the radar.

"What keeps you going as a writer is keeping everything small," the gregarious New Yorker said recently. "I don't have a manager, a publicist, none of those things. And I don't have the pressure. All I have to worry about is staying honest, and that's where hit songs come from. When Carole King did Tapestry, that was some real sh--. There wasn't anyone calling her up going, 'We need hits.' "

DioGuardi grew up in the city with one side of her family involved in the arts and the other involved in business. Initially, she chose the latter, studying pre-law at Duke.

"I was in a totally different place, but I wasn't happy doing that," she recalled. "I always sang, so after I graduated, I decided I wanted to be a singer. I never wanted to be a writer or producer. I joined a garage band, and my parents were freaking out. I wanted to put together a demo, but no one would give me songs. I don't know if I was bad or what. So I decided to write. That's why I got into it."

To fund her recording sessions, DioGuardi got a job at Billboard magazine as the assistant to editor-in-chief Timothy White (who has since passed away). "I would demo over my lunch break and go to the studio every night and still have to be at work at 9," she said.

Still, the job paid off, as it taught her the music business from all different sides. Eventually, her group signed to MCA, but was dropped before anything was released. Later, a demo landed in the hands of Paula Abdul, who requested her assistance on a track called "Spinnin' Around."

Abdul's album also never saw daylight, but their collaboration landed in the hands of Kylie Minogue, who scored a massive hit with "Spinnin' Around" in Europe that revived her career and launched DioGuardi as a songwriter.

Soon after, DioGuardi met producer Steve Morales and worked with him on Enrique Iglesias' Escape. That collaboration led to projects with John Shanks, Scott Storch, Walter Afanasieff, Guy Chambers, Cory Rooney and many of pop music's busiest producers.

DioGuardi has since carved herself a niche with young, usually female singers, although she says her songs for the likes of Ashlee, Hilary and Lindsay have come together in different ways.

"It really depends," she said. "I wrote 'Come Clean' with John [Shanks] and we changed some of the lyrics for Hilary. 'Pieces of Me' we all did together. 'Screwed' for Paris [Hilton] was something we pitched to her.

"Sometimes I just write them or we write together. I try to spend a lot of time with them and get in their head. Like Lindsay and Ashlee are really feisty, so I try to make them feel different.

"It's not like Nashville, where we're like, 'Hey, let's write about the sun today,' " she continued. "It's like, 'How are you feeling today?' And we just try. Some are good, some are bad. Like with Lindsay, 'Over,' we wrote about Wilmer [Valderrama]. We try to make it as pure as possible."

After Ashlee was caught lip-synching on "Saturday Night Live," DioGuardi was accused of not just writing songs for her, but singing them on her Autobiography album.

"Ashlee can sing," DioGuardi insisted. "Did I help in the phrasing? Yes, because I wrote the song, but it's not me singing. She's evolving as a singer. These are young girls. The more they do it, the better they're getting. They can sing. They all have ability, but there's a lot pressure on these girls. When I haven't sung for a while and I sing, it sounds like sh--. You have to realize that even amazing singers are tuned and tweaked. Celine, everybody. Because radio's used to hearing that. I was listening to a Madonna record and the pitch was horrific. That would never happen today."

DioGuardi is quick to defend her clients, although she also pokes fun at that world. Recently, she started getting record-deal offers herself, but she turned them down because she "thought it would be really cheesy to do things that were solo girly things, especially since I don't have a TV show or a Dermasil commercial."

DioGuardi is hoping to write again with Ashlee as well as all of her past clients. In the coming months, she's working with Santana, the Pussycat Dolls (as a co-producer) and Hilton.

"She's actually got a really interesting tone and she has an amazing presence," DioGuardi said of the hotel heiress. "She brings a life to everything she touches."