Comic book writer Kurt Busiek penned a great essay on breaking into the writing business. His main point is that most successful people make their own paths without following the rules, many of which (like submission guidelines and such) exist to weed out people rather than usher in new ones. Also, that "overnight successes" usually aren't. He writes:
You don't need a map. You need to figure out what you've got and capitalize on that. Instead of bemoaning the fact that you can't mail in something that a publisher wasn't going to read and wasn't going to buy, try to figure out something else, something that builds off your skills or knowledge or contacts or whatever . . . Make comics, not just proposals—even making crappy comics that convince you that you never want to draw another background again in your life will teach you more about telling a story in pictures on paper than a zillion proposals in the slush pile.
Kurt's a zillion times more experienced and successful than I, but my observations match his. He offers examples of how colleagues of his got published--each unique, none following any formula. Same for the writers I know. When people ask me for advice, I sincerely don't know what to say. I can describe how I did it, but that's nearly useless because it's not replicable for anyone else in any other place or time. I'm reminded of Carl Sagan's line, "If you wish to make an apple pie from scratch, you must first invent the universe." If you want to do it my way, you're starting 30 years too late.
All I can suggest is: First, do the work. It's surprising how many "writers" never actually write and how many "artists" never draw. Having an idea for a thing is not the same as sitting down and creating 20 or 200 pages of an actual thing! The latter is orders of magnitude harder. Then get it out into the world any way you can. Print it up, show it to people, send it to editors, put it on a website. Find communities of people doing the same thing--easier than ever on the Internet--and share work with them. One of those people in your favorite web forum could be looking for a writing partner or content provider five years from now. Plant a hundred seeds and two or three will bloom--and they'll probably be the ones you least expect.
Also, don't take without giving back in return. Don't expect or demand favors. No one owes you anything. Few people are as tiresome as a newbie who pops up to announce, "Hey, everyone look at my great stuff," argues with well-intentioned critiques, insults pros whose work is obviously worse than theirs, and then vanishes into a billowing cloud of entitlement. Bad form, and people have long memories. If you're polite, thoughtful, professional, and take the time to show others you care about their work, some will bend over backwards to help you.
While determination is important and admirable, I'm not a huge proponent of the "winners never quit" school of success. Busiek writes about a guy he sees at every convention who's been trying to break in for decades and will simply never be good enough to make it. I know people like that. I've also had times when I've wondered if I'm that guy. It's a fine line, deciding whether to keep slugging away versus recognizing that maybe that brick wall isn't budging and you're better off trying another angle or something else entirely.
My criterion is external evidence of progress. At first, probably no one will respond to your stuff at all. Then maybe you'll get a nice note from a stranger. Then maybe some encouragement from a pro. Then maybe an editor invites you to send some material. You sell a little thing to a little client, then parlay that into selling a bigger thing to a bigger client. Look for signs of advancing toward a goal rather than running in place.
The last time I touched on this, Friend-of-the-Blog Mike Peterson replied with a quote by Mark Twain that I can't top (and Twain's entire essay on the subject of advice for aspiring writers is funny and worth reading):
Write without pay until somebody offers pay. If nobody offers pay within three years, the candidate may look upon this circumstance with the most implicit confidence as the sign that sawing wood is what he was intended for.