I started writing seriously (that is, “for money”) when I was in college. For me, the keys to mastery were:
Write a lot. I became much more fluent during the course of my first book, Through Dungeons Deep: A Fantasy Gamers’ Handbook, (Reston Publishing, 1981) which covered how to play Dungeons & Dragons and role-playing games in general. When I started out, I set myself a quota of 1,200 words a day and just couldn’t do it. At the end, I’d upped my quota to 4,500 and beat it every day. (I have since had a number of 10,000 word days.) Going over my old work, it seems that the extra speed was a free bonus, involving no loss of quality.
Big works are easier than small ones. I think that’s it’s infinitely easier to write a 100,000 word novel than a hundred 1,000-word short stories or even four 25,000-word novelettes. Similarly, it’s easier to write a nonfiction book than a series of articles that add up to the same length. Coming up with new themes is harder than running with what you’ve got.
Write for a reason. I come from a storytelling tradition, which means that connecting with my audience is important to me. If I lose them, I’ve screwed up. I also wrote for money from the beginning, because I was broke. Writing is hard, so you need a goal in mind.
Writing is hard. It’s harder than anything. After a hard day’s writing, I sometimes lose the power to speak coherently. If that happens to you, you’re doing something right.
Pick up the nuts and bolts as you go. Perfectionism is for editors. Just keep going. Get to the end before you rewrite. Keep notes, but leave the earlier passages alone. A lot of people use perfectionism and revision as an excuse to never finish anything — or to never start. There are editors everywhere, so your stuff can be professionally washed, waxed, and detailed after the fact. So get to work!