To have a sense of humor
Go ahead, insert jokes about humorless feminists here. But even allegedly unfunny feminists acknowledge how extra-dry fighting sexism can be, and so we hope that when men join us, they, too can have a good, not always so self-serious laugh about gender roles and the complications in working to level the playing field.
To be pissed off, too
That said, it's nice when a dude can see how utterly unjust the way women are still treated the world-over, and get a little pissed about it. Bikini Kill's Kathleen Hanna has a funny story about joining Cobain, her "angry young feminist" friend, once to vandalize a fake abortion clinic and spawn the infamous lyrics to "Smells Like Teen Spirit." I'm not endorsing vandalism, just a willingness to shake the sexist snowglobe a little.
To not get hung up on the name
You don't have to call yourself a feminist to be welcome at this party. Not every dude is going to fly the feminist flag proudly, and that's totally cool (not to mention, lots of kickass women don't identify as feminists either). I knew a rather enlightened fellow in college whose girlfriend was frustrated that he refused to identify as feminist, in spite of him claiming to see the very injustices she was so upset about. "I'm a humanist," he insisted. "I think everyone is discriminated against." He has a point. Cobain was also a humanist, and while there's a separate argument about the semantics of change that is totally worth having, what really matters to men and women is walking the walk.
To really mean it
We don't care how you got here, as long as you mean it. That means no sensitive ponytail man schtick to get more 'tang. I'm sure more than one woman has met a male feminist who seems a little too preoccupied with our safety, a little too willing to jump in and rescue us, a little too into the narrative of the vulnerable woman and the man who's here to show her he's not like those "other guys." Gross. Women need men who want to work as our equals and helpmates, not our protectors and guardians. And especially, don't be that guy who had daughters and is now the most enlightened feminist man on the planet about injustices toward women. Unless it's actually true, and you're not just saying it, and the experience really has changed how you treat actual women in the workplace or in your personal life.