Some randon musings...
It's no joke that writing is the best practice for writing. Prior to publication, I joined an RPG mailing list and started commenting at length just for the practice! Banging out 1-2k words on unrelated/converstional e-mail is still my warm-up exercise for work each day. If you want to make a living at it, you will need to achieve some awesome speed. These days I generate about 1,000 presentable words an hour. But... I only write for about 2-3 hours a day if I can help it. You also need to take time to think about your text. I often spend as much to about double the amount of time spent writing just brainstorming or musing over it and editing in my head. That's half-a-million words in print and better than 20 books talking

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Recognize the different skill sets involved in RPG writing. Organization, editing, basic writing, fiction writing, and game design are all different abilities. Any and all of them can be improved by study and practice. You can be deficient in one or two, maybe, but you should try to improve your craft in all of them, not just the ones that caught your fancy when you started. Proofreading your own stuff is also an essential skill. Save those editors pain at every opportunity

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Be aware of what you are writing for. All of my hired work starts with "This is a piece of a product that must sell". If you are working for money, you have to create something worth buying

. As you gain experience, that simple maxim has progressively more and more profound effect on how you approach a project...
Lots of good advice already, but do not underestimate the importance of the shmoozing skill set. There is a lot of talent out there, so learning how (and where) to show off yours is an essential step. As someone who now hires freelancer from time to time, I'm not interested in specific ideas, or enthusiasm so much as a good demonstration. People who tell me "I want to write" get a half grin and a "that's interesting..." look most of the time. People who write 4-5 pages of good stuff for a game I'm working with and ask "Wadda ya think?" catch my eye. After doing that 3-4 times then there's fate. Those people are the ones I remember and are far more likely to get a letter saying "Hey, I've got a hole and I think you can fill it. Wanna give it a try?" than someone who tells me "I have this great idea" but doesn't do the work of writing it out
before comming to me. Even an unpublished writer can (and should) build a portfolio.
Another thing to consider is that there are going to be days, weeks, and sometimes months were its not fun - its work. Getting crushed by a deadline or having to step in and clean up someone else's mess sucks. Hitting a writer's block for two hours sucks (two weeks is kinda life threatening..). If you don't plan to love it, honestly, don't even start. You can still save yourself

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Do not make light of deadlines. Rule number one is after making an intelligent and educated estimate of how long in a reasonable universe something should take - double it. Tripple it if you're doing it for money. Its that simple

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Hope this helps,