Girls in gaming is always a hot button topic, but rarely do you find the opinions of the people actually affected by sexism in the gaming community, such as this tumblr written by Retired Super Smash Bros. player Lilian "Milktea" Chen.
Go read the article, but for the TL;DR people, it boils down to - treat people like people. Don't assume that girls are trash, don't sandbag against them and deny them a good match, don't place additional roadblocks in their way from joining the community.
That said, I really do have to take issue with some things. I play Hero Academy pretty competitively, and am on the Hero Academy fan Facebook group. There are a few girls playing HA, but by and large, they do not take the game all that seriously, and they don't even seem to want people to take them seriously. While some of it might be, as Milktea says, young girls buying into what's expected of them, I suspect that that's not 100% the case, and that Milktea might have her experience coloured by what she wants out of it (i.e. be considered to be a tournament level player).
Is it really so wrong to want attention by playing a game? I don't think that, in and of itself, being attention seeking, and being known for being an attention seeker, is all that bad. I mean, Juicebox was famous for doing the Juicebox dance whenever he hit an Ultra. Even though he never broke 9th place, everybody knew about him. Is it that bad to be famous for doing something silly? Pretty much all of cosplay is being attention seeking for attention's sake.
I suppose it might be a downside to be tagged as "just another attention seeker" just because of your sex, but I think Milktea goes to far in the other direction - attention seekers sometimes just want attention, and there's nothing wrong with giving it to them. People like being famous, and if you can't be famous for being good, hey, being famous for looking good works too.
If you are a girl gamer, do you feel pre-judged for being female? As a guy, do you catch yourself pre-judging people based on their sex? Do your interactions with people change based on whether the person you're interacting with is of the same or opposite sex?