If your only goal is to write for Marvel or DC, I don’t really have any more advice other than to constantly write well-thought out pitches. But if you just want to break into comics in general, you could write stories featuring your own characters. Try to get something published by an indie publisher, or (if you have the funds) self-publish something. I bet the more work you have in publication, the more likely Marvel and DC would notice you. Just imagine going to an interview and handing your interviewer an actual hardcopy of a comic you wrote, as opposed to just telling him about some ideas you have.
Bendis toiled in the undergrounds for years until Quesada discovered Goldfish and Jinx. Of course, I’m sure it helps to know someone in the company to help get your foot in the door.
Anyway, working in the comics industry is probably a job that pales in comparison to being an Internet smartass. At least, that’s what I tell myself whenever I am too lazy to blog.
This is excellent advice. Get established and make your name noticeable. Think of all the guys who write for the big two houses at the moment: they’re veterans. They’ve been in the business long enough to have their chops down and consistently turn in material that meets at least the basic level that’s expected of them. The more professionally you’ve honed your skills and the more noise you make in the most far-flung areas of the industry, the closer you’ll get to the big guns.
I’ll also drop in a little wisdom that Eisner and Miller kicked around in the book (oddly titled) Eisner/Miller. If you’re going to write for the big properties, you have to be at least two of three things: really terrific, on time, and/or easy to deal with. It doesn’t matter if you’re the second coming of Jack Kirby; they’d rather bring in a decent craftsman with a professional work ethic than a brilliant artist who’s a shithead.