You know us and love us, whether you realize it or not. We struggle alongside you, often on issues that touch your lives far more than they touch ours. As we have supported you, it is time for you to support us. Use the word lesbian often. Think about your own sexuality.
Much homophobia is rooted in people's discomfort with their own feelings for people of the same sex.
Don't always claim heterosexual privilege by making it clear you are straight. Try casting doubt in people's minds about your own sexual orientation every once in a while. Make it clear that it would be no insult to mistake you for a lesbian. Refer to past relationships with women, if you're had any. Talk about how any issue under discussion might affect lesbians.