The mentoring mandate

And when it comes to bringing and keeping women on board, there's no substitute for woman-to-woman mentoring. According to the Catalyst study, women who list another woman as the most helpful person in their careers are more likely than other women to have reached the top one or two reporting levels in a company. "Thus, while it is true that support and mentoring by higher-level executives -- both male and female -- are essential to developing leaders, we find that women mentoring women is of special importance," the study says.

"I consider it part of my job," IBM's Carter says. "If I hadn't had mentors, I wouldn't be here where I am today."

And it's mentoring programs that are formed organically, rather than those mandated by HR, that are most successful.

"Organizations that seem to do the best tend to have mentoring programs that are initiated by, led by, and participated in by women leaders in the company," says Baxter's Terrell, who spent several years working in the male-dominated auto industry before signing on with her current employer. "When women are supported by the senior line managers of a business, that's when you see things take off."