Almost all Fortune 500 executive boards now include at least one woman, and many have two or more. The average board among these companies is made up of nine men and two women — double the number of female board members in 2006. While these gains are promising, they appear to be slowing. Last year, the overall number of female corporate directors in the Fortune 500 actually dipped slightly.

Why the apparent flattening?  A report from MIT / Sloan shows that the reason women aren’t making more rapid inroads is that few have reached the most influential board leadership positions. Although more women are on boards now than 10 years ago, very few have been promoted to a post that would give them influence beyond their seat at the table.

“There are three layers of progress for women,” explains former Xerox Corp. CEO Anne Mulcahy, who currently sits on several Fortune 1000 boards. “There’s the breaking in part of getting onto boards. Then there is the critical mass part of getting more than one woman on each board. And then there is the influence part of getting women into leadership positions where the real power resides.”

Read more on how you and your mentor can work to help you achieve true influence.