I get asked this question a lot, so I'll give my pat (but serious) answer:
Find yourself an editor.
That is, find someone who'll take what you think is crystal clear, engaging prose, and turn it into something that actually is crystal clear and engaging. I couldn't do my job without my editors. (For those who don't know me, I work as a freelance mathematics writer. I report on developments in mathematics for publications such as Science magazine and SIAM News.) It's very easy, especially when writing about a subject you know inside out, to fool yourself into thinking you've explained things so that any fool can understand it; it's equally easy, when reading someone else's writing, to notice they haven't explained things very well. A good editor will, at the least, point out your failings. A great editor will fix them.
Now if I can only find someone to edit this posting....
