I know the question was posed at a conference about women in technology, but I think a good answer to the question of how to ask for a raise goes beyond narrowing the pay gap between women and men. There are plenty of men, myself included, that also feel like we're not getting what we're worth and what our colleagues are getting, but don't have the knowledge how or courage to confront our manager and ask for a raise. Perhaps women are more predominately or more severely affected by this issue, but there's more to it than the men/women pay gap.
I liked Maria Klawe's answer a lot more, it provides some really useful advice beyond "if you think you're being payed less than your male peers, ask for a raise".