Here's a quick essay by David Ogilvy (legendary marketer) on how to write:
Here's the pieces of advice I like the most from this:
2.) Write the way you talk. Naturally.
3.) Use short words, short sentences and short paragraphs.
4.) Never use jargon words like reconceptualize, demassification, attitudinally, judgmentally. They are hallmarks of a pretentious ass.
8.) If it is something important, get a colleague to improve it.
9.) Before you send your letter or memo, make sure it is crystal clear what you want the recipient to do.
10.) If you want ACTION, don't write. Go and tell the guy what you want.