Minnesotans will be voting this fall on whether to amend the constitution to define marriage as "between a man and a woman". To do our bit to defeat that, Emma and I are volunteering at the ACLU booth at the Minnesota State Fair. We just attended a training session where they made an important point: most of the arguments that are convincing to the supporters of gay marriage are not convincing to swing voters.
Nerds and wonks like me get obsessed with facts and principles, but if you don't already think this is an equal rights issue, being told that it is one won't sway you.
What sways people is making it personal. Don't focus on the legal aspects. Focus on the human side of marriage. If you're gay, talk about wanting to be able to make a public commitment to someone you love. If you're not gay, talk about gay friends or relatives who have married or who want to marry. Ask the other person if they know openly gay people—most Americans do—and ask if they've talked with them about whether they would like to be able to marry the person they love.
If you're on the left and you're talking to someone on the right, don't assume they're opposed to gay marriage. Neither of the major parties has taken a position on gay marriage, and its supporters include the Log Cabin Republicans. Politics are irrelevant in this issue. Love is what matters.
One way to support the right to marry is to support the ACLU. They have a four-star rating at Charity Navigator Rating: American Civil Liberties Union Foundation.
See also: Debunking marriage myths :: ACLU of Minnesota