What a great question! It has been a combination of hard work, taking chances and networking.
To grow your writing career, no matter who you are working for, you need to produce quality work and to meet your deadlines. If you accept a 21,000 word contract due in three weeks, you darned well better meet that demand. If you cannot do it, do not accept the contract. It is as simple as that. Do not bite off more than you can chew.
I have met every deadline I have been given. It is not an easy thing but it is one way to prove that you are someone worth contracting and trusting with important assignments. Once your editor trusts you and knows how you work, they will be happy to vouch for you.
The way I started working for Catalyst Labs was my editor, Sean Everette, turned to the green haired man he was talking to and introduced me to him as "This is Jennifer Brozek. She is one of my best freelancers. She has an interest in writing for Shadowrun." The green haired man turned out to be one of the lead editors for Shadowrun. He shook my hand, handed me a business card and told me to email him after the convention was over.
It was just that easy. My editor believed in me and vouched for me to another editor and the ball was rolling. My hard work paid off.
The next thing I did was to take every chance and opportunity I came across – Every RPG contest that I qualified for, pitched RPG ideas into editor slush piles and for those people I already knew in the industry, I asked them for pointers and for work.
Taking chances comes with a high rate of rejection. All authors should expect rejections and be hardened to it. Editors reject your work because it is not what they are looking for or need. It is not to tell you that you are a bad person. It means you did not qualify. However, if this disqualification and rejection came from poor technical writing skills, you need to improve those skills immediately.
The hardest thing to do is to go to a friend and ask for work. But freelancing is all about going out and getting the work before you do it. Most people who work in the RPG industry are freelancers. It is not a rich industry to work in. I do it because I love it. Not because I want to get rich. (Well, I do want to be rich but I'm smart enough to know that it won't be through my RPG writing.)
The next hardest thing to do is to pitch product ideas to the editors you want to work for. I was in a panel at a convention about "How to write for White Wolf" at Gen Con one year. After the convention, I emailed Eddy my product idea. I know I went in to the slush pile. But, a few weeks later, I got an email back from him approving my idea. I had followed the pitch guidelines to the letter and it got the Editor's attention in a good way.
If you have an idea, you must submit it. The worst they can do is say, "No." If you do not submit it, you are telling yourself "No." for the editor and that is not fair to either of you.
The first time I thought about networking, it gave me hives. As it turns out, I'm good at networking. Really, all it is, is talking with people in the industry at events that are designed for the industry. Gaming conventions are perfect for this. There are some obvious tools of the trade that you should have with you but mostly, it is getting some face time with other people, letting them know your availability and giving them a way to contact you or getting a way to contact them.
Business cards are invaluable for this. It is how I ended up writing for Rogue Games on Colonial Gothic. I had a conversation with Richard at Gen Con and let him know my interest in writing for him. I also put out my previous credits and gave him my business card. Then, and here is the important part, after the convention I contacted him. I reminded him of our conversation and reiterated my interest in writing for him and his products.
Editors frequently meet dozens, if not hundreds, of authors (or wanna-be authors) at conventions. It is hard to keep all of that in mind. Authors also meet dozens of people in the industry. When you take a business card, take the time to flip it over and make a note of what you talked about and why you are contacting them after the convention.