I have been pondering, therefore, a short answer to the question that covers every important issue in as brief a time as possible. The points below represent my attempts to do that.
I'm posting this here because I'd love to know how others approach this very question, and if ways exist to condense the answer even further.
Here's my list.
1. Read a lot.
2. Write a lot.
3. Write what you love but be aware of the market.
4. Define your version of success and take concrete steps towards achieving it.
5. Be professional at all stages of your career.
6. Listen to everyone.
7. Be visible.
8. Challenge yourself, always.
9. Never believe you've figured it out, because everything changes.
10. Work hard.
ETA: there may in fact be a Zeroth Commandment!
ETA: see the comments to this post for an expanded version of this list, as published in the SA Writers' Centre's Southern Write newsletter, December 2006. Or just click here
ETA: a similar article has been published at Writers' World, here.
ETA: also see the A-Z of Writing.
That's still not a very short list, and it doesn't include anything explicit about community, which I think is very important--but I suppose that being visible and listening to people kinda assumes that you're interacting with someone.
I figure that if people are doing all or most of the things on this list then they're substantially increasing the likelihood that they'll achieve the goals they set out for themselves. Points 3 and 4 particularly relate to someone who wants to write for a living, but I think they're still important general points. The hardest thing, arguably, is not being published but working out what kind of writer you could be (there's likely to be several different answers to that question) then seeing whereabouts in the market you could fit. The last part is difficult, as the market is always changing, but you can't ignore the question or you could end up consistently barking up the wrong tree. Or a whole forest of wrong trees, until you stumble across a right one by chance--and who wants to leave something as important as this to chance?
The flipside of 4 is the awareness of what constitutes failure as well as success. When I started writing seriously, I gave myself ten years to have a book published. If I didn't make that deadline, I swore that I would give up and try something else. That gave me a sort of anti-deadline--something to strive against rather than for. I think that's important too. (Update: See Jay Lake's post here on the subject of quitting.)
Anyway, that's enough from me. I'd love to hear people's opinions on this. Save me from oversimplification and protect me from proselytising; whatever is required. :-)
ETA: also see Robert J Sawyer's "Eight Things New Writers Need To Know".