My best advice on learning how to write well would be to read. Read when you're on the bus. Read when you're waiting in line. People have a lot of time in a day where they are waiting. Next, as Morendil writes, is to write as often as possible. Describe the scratched, faux wood desk that your laptop/desktop is resting on, or the delicate webbing on a dried leaf and be sure to have people critique it. If you're writing fiction, have someone read over your plot first, and make sure it's someone who reads a lot and is a critical reader. Chances are they'll be able to spot plot holes or inconsistencies that you might miss. Also, take a break from something for a week and then go back to it. You'll find a slew of "I could have worded that better" -s. Most of all, be clear, concise, and passionate about what you write. If people see the author has no passion or apparent interest in what's written, they will be less inclined to read past the third or fourth paragraph. In fiction, if you are writing about a real place (for example, a character lives in Ischia, Italy) look up information about the place. Not only should you have the general attitude of the people down, but the foods, times people generally do things, local sayings, etc. (Like the majority of people living in Ischia take naps around noon because of the heat.) I am babbling, and will stop now, but I agree on the feedback. You may think you are getting your point across, but does everyone else?
The ability to write efficiently and persuasively is important in many areas of life, and especially for spreading rationalist memes and hence raising the sanity waterline.
While there are a lot of very good and persuasive writers of both fiction and non-fiction on Less Wrong there seems to be relatively little advice on how to improve one's writing skills.
While there are a huge number of writing guides available, much like general self help they rarely reference studies on the effectiveness of the advice contained, and while some come from very successful authors, the problems of generalising from one example are well known.
Given this, would people be willing to supply rationalist supported strategies for improving writing skills?
Notes,
I've looked for previous posts on this subject, but if I have missed a previous good discussion please link to it and I will close this thread.
The most obvious piece of advice would be to engage in large amounts of writing practice, but hopefully you will be able to supply some more strategic advice than that.
Edit,
Consensus so far is that a high level of practice is very important, ideally paired with useful and continuous feedback. Otherwise a general agreement that the process is very idiosyncratic, with a few good suggestions for resources that have worked for individuals.
Ideally we'd be looking for advice that has helped a large majority of people to have tried it, if any such exists.
(Also added links)