The best pieces of advice I got about this were (I think these apply equally well to technical and nontechnical writing):<p>1) Write a lot. At first, write without any intention of showing it to others, and without any judgement about how good or bad it is. Just write.<p>2) Sit down and analyze examples of good writing and what techniques/habits they employ.<p>3) At first, emulate the style of authors you admire. Your own style will emerge later.<p>4) Recognize that most of what results in a piece of good writing is good editing.
It was in the context of writing fiction, but Stephen King (who knows a thing or two about writing) said - in his book <i>On Writing</i> - something like "the best way to get good at writing is to read a lot and write a lot."<p>I'd actually recommend reading that book, even if your interest isn't fiction (and definitely if it is). There's some good stuff in there.