One of my favourite pieces of writing on writing that I have ever read is by Paul Graham. Paul is a fascinating entrepreneur. He designed the algorithm used by most anti-spam software around the world and he now runs Y Combinator, a seed funder for startups. He is also a superb essayist.
In this short essay, which he says he wrote accidently while answering an email, he gives his best advice on writing. (The bold selections are mine and represent some points I know from my own experience to be very powerful.)
I think it's far more important to write well than most people realize. Writing doesn't just communicate ideas; it generates them. If you're bad at writing and don't like to do it, you'll miss out on most of the ideas writing would have generated.
As for how to write well, here's the short version: Write a bad version 1 as fast as you can; rewrite it over and over; cut out everything unnecessary; write in a conversational tone; develop a nose for bad writing, so you can see and fix it in yours; imitate writers you like; if you can't get started, tell someone what you plan to write about, then write down what you said; expect 80% of the ideas in an essay to happen after you start writing it, and 50% of those you start with to be wrong...