Recently, DC Comics put out a (rather condescending) notice that it's trying to attract more female readers. There are a whole lot of women reading mainstream comics, but they're very much in the minority. Anyway, here is my response. It's largely made up of points made by other writers, but collected under a single header, with my own commentary and ideas.
1. The first one is the most obvious: Stop writing female characters as sex objects. There's nothing wrong with writing about a sexy woman as long as she's sexy on her own terms, so stop putting female characters in positions that "just happen" to show off their physical assets. Stop putting them in peril so the male characters can rescue them (why the hell does Mary Marvel get bound and gagged so often if she has the same powers as Captain Marvel, anyway?). And give them sensible outfits: It's okay for a female character to like how she looks and show off a bit, but what woman in the world with any ounce of self-respect would wear this abomination?
2. Also, stop using sexual harassment or romance as motivations for female characters to become heroes -- it's possible for women to do heroic things that aren't motivated by love or fear of/revenge for harassment. Don't have her trying to live up to her father (Lara Croft) or other male characters (Supergirl). The problem with all these things is that they tie the men in her life directly into the woman's story, rather than making her her own character.
3. We all know that mainstream comics are basically about adolescent male power fantasies. This isn't in and of itself a bad thing. But I ask you: Where are the adolescent female power fantasies? Many young girls dream about being a beautiful magical princess (and I cover that below), but many girls also would like to punch out whoever bothers them, perform amazing feats to the wonderment of onlookers, and just do good deeds for their own sake. And yet many of the best-known and longest-running female super-heroes are just lesser versions of male counterparts: Supergirl, Batgirl, Mary Marvel, etc. Even Wonder Woman, the greatest super-heroine, is not as strong as Superman (I like to think that her formal warrior training and magical weapons bring her up to his level, but still). That's the biggest problem with women in comics: They're strong, but never as strong as the men.
4. In the same vein, less female super-heroes with passive powers, like the Invisible Girl or Shadowcat from the X-Men. Admittedly, invisibility or walking through walls could be really cool in certain venues (Sue Storm would make an awesome secret agent, for instance), but not in mainstream comic books. There need to be more female super-heroes who can bench the Empire State Building and shoot frickin' lasers out of their eyes and such.
5. Combining items 1, 2, 3, and 4... Market the hell out of Wonder Woman. Seriously, Wonder Woman is the archetypal female super-hero. WW's one of the "Big Three," but her lack of mainstream exposure compared to Superman and Batman is inexcusable. Most people know her only from the campy '70s TV series with Linda Carter. Diana also has only one comics series, compared to Clark and Bruce, who each have three -- if we can have Superman and Action Comics and Batman and Detective Comics, why not bring back Sensation Comics? There needs to be a Wonder Woman movie franchise, animated series, lunchboxes (they still make lunchboxes, right?), underoos, etc., etc., etc. The public should know her story inside and out like they do Supes and Bats. Make people know her, love her, want to be her.
6. Get more female writers and artists. Really, women just tend to be better at writing women then men because, well, they are women. This is not to say that men should never write about women of course, just that female writers have a built-in perspective on the subject that male writers lack. Come on, Gail Simone can't do it all herself.
7. Create an imprint for importing Japanese manga, including many shoujo titles. This one's a no-brainer, and DC's already doing it with their CMX imprint. The biggest problem I see is finding any really good manga titles before the existing American publishers gobble them all up and glut the market.
8. Publish more comics in the trade paperback format and sell them in bookstores rather than monthly 22-page pamphlets in comic book shops. Not many girls go to comic book shops for reasons that are pretty much obvious: they tend to be dark, dirty, and frequented by creepy male nerds. As such, they just can't easily reach female readers. (The things comics shops can do to make themselves more female-friendly is a topic for another time.) But if major companies were to publish their comics in the same format and put them one shelf over from Inu-Yasha, a lot more girls would take notice. Again, DC is already doing this, with their (poorly-named, IMO) Minx imprint.
9. Bring back Amethyst, Princess of Gemworld. Not all female heroes need to be about smashing things. As Neil Gaiman noted in The Sandman: A Game of You, normal girls who find out that they're really beautiful princesses are a very important archetype in fiction, going back at least as far as Cinderella. There is a reason that Sailor Moon was so popular, after all. Amethyst would be a perfect gateway for young girls interested in comics, and the fantasy setting could attract even more readers from different demographics. And hey -- princesses and power fantasies don't have to be mutually exclusive. Amethyst could be a magical princess who kicks righteous ass. Why not?
10. Three words: Green Lantern butts. Okay, okay, I'm kidding. But really, a little "manservice" for the ladies now and then wouldn't hurt. Just use it sparingly, because while it's good for an ironic chuckle now and then, it gets old fast.
So there you go.