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When to say no to a board position

October 4, 2011, 5:43 PM UTC

Archer Daniels Midland CEO Patricia Woertz and BET CEO Debra Lee

With only 15.7% of Fortune 500 company board members being female, there’s still plenty to do to get more women to the table. But what happens when you’re already there? For Archer Daniels Midland CEO Patricia Woertz and BET CEO Debra Lee, who chatted with Fortune senior editor Jen Reingold about “Building Your Personal Platform” at the Fortune Most Powerful Women Summit, vetting the countless offers they get to join various boards is an issue.

Both women agree that it’s important to build personalities outside of their job descriptions. “An external profile protects you from the politics of a company,” says Lee, who adds that joining boards has allowed her to not only build new business relationships, but also explore her personal passions – like politics (Lee is on President Obama’s management advisory board). Woertz joins boards that touch on her passions, but also help ADM (ADM) – like Procter & Gamble (PG).

Though Lee and Woertz are no doubt pros at multi-tasking, they struggle to balance their board memberships and running their own companies. The solution? Sometimes, you just need to say no. “No is a complete sentence,” says Lee. Woertz agrees, adding that you need to “choose your board; don’t let it choose you. Pick carefully – and don’t overdo it.”

Lee, who was on Genuity’s board when it went bankrupt in 2002, learned that lesson the hard way. She was told the company was going to go public and bring her millions of dollars, which seemed like a pretty great deal. But the company went public in 2000, didn’t bring her millions and went under two years later. The take-away: “Be careful. Know and trust the company’s CEO and management team.” Also, “Don’t be afraid to follow your gut.”

So what advice do these women have for rising stars interested in joining boards – and creating their own personal brands? Self-promotion can be tough. You don’t want to come off as over-zealous, but you don’t want to fly under the radar. It’s different for everyone, says Woertz. “Work within the culture you know. Understand what the right behavior is for your company.”

Check out our additional coverage from Fortune’s Most Powerful Women summit.