In the aforementioned book by Bendis (who has been writing comics since the 90s), he interviews noted editors on what advice to give to aspiring creators if they meet them at conventions:
"TOM BREVOORT (Avengers): Nothing succeeds like the work itself. I know a lot of people in and around the industry, and a lot of people who are struggling to break in, or to stay in. And there are some terrific people among that group. But none of that matters once it’s time for me to hire somebody to do a job. At that point, it all comes down to how good you are on the page, right now—not how good you were years ago or how good you might one day be. There are other factors, too, of course—reliability being the very first one. But the most reliable guy in the world won’t get called up if the work he’s producing doesn’t have that spark."
"SANA AMANAT (Captain Marvel, Ultimate Spider-Man): Research the books I’ve worked on and angle samples accordingly. Editors tend to develop a style themselves as they grow, so if you think you’d be a good fit for the writers or artists we tend to work with, say so and show examples that prove it. Further, editors generally have certain books (Spidey books versus X-Men books, etc.), so send samples that might work for a particular title we work on. We’re constantly on the lookout for new talent, but when you target our books or styles specifically that actually helps us do our job."
"DIANA SCHUTZ (Dark Horse Comics): I recommend establishing an online presence. A lot of people are doing comics online. In comics, as in any other field, getting your name out there is important: networking. Keep your proposal short and concise. As an editor reading submissions, I can tell within the first page or two whether a proposal is worth pursuing or not. And your writing another ten, or a hundred, pages is not going to convince me otherwise. You need to put your maximum punch right at the beginning and sell me on the project by page 1!"