|Oddman, the best way to pitch is to keep it short and keep it simple.|
Some companies have very specific guidelines they want you to follow, but generally the best advice is:
Try to keep it to no more than 50-100 words per page, probably no more than 10-12 pages. Generally give the jist of the story, jist of each of the characters, and an idea of where its going, and a few pages of art.
You want an artist. Marvel and DC, to my knowledge, aren't hiring people who don't already have some writer's cred (Dan Didio actually said to me this weekend that he doesn't want people coming to DC to get their feet wet) with published or produced works.
Diana Schutz suggests getting your name around by drawing little minicomics (even with stick figures) and handing them out to editors at conventions. Creates name recognition and some interest in your work. Also, a decent writer doesn't really need anything more than stick figures and a little line drawing to get the story across (seriously...it doesn't sound like you could, but you'd be surprised).
Beyond that...I really can't say. I've spoken with some editors and some writers, gotten advice from people and generally been planning for the past year. I'm not published, yet, nor am I an editor, but this is what I know from what I've heard from folks I've spoken with.
Oh, and B. Clay Moore said that Image has the best guidelines on their site, and that submitting Image-guideline-friendly stuff will get you looked kindly on by most companies. I have no idea how much truth there is to that, but I felt it was probably worth noting.