Keep it as brief as possible. What will you deal with most quickly, the three paragraph email or the three page one? Your reader will do the same.
Choose familiar words, for example, buy rather than purchase, and words with concrete meanings (car, not mode of transport). Overly abstract words can hide your meaning.
Avoid unquantifiable quantifiers: how soon is soon? How big is big? Be specific: say by Thursday, not at your earliest convenience.
Don't bury your message in waffle. Get to the point quickly. Practise pruning your prose!
Are you over-using the passive voice? You want to aim for about 30% passive sentences, fewer if you're writing things like procedures or instructions. It's OK to use the passive voice in meeting minutes or academic papers.
Ask yourself what your readers need to get from your writing, or what you want them to do as a result of reading it.
Everything you write should work towards this. Choose examples and language that is meaningful and relevant to your reader.
Check your facts and check the mechanics like punctuation and spelling. Getting it right makes you look professional and thorough.
Each mistake causes your reader to drop their estimate of your competence, and gives them an excuse to reject what you're saying.
Think about how you react to a slab of solid text – or to something laid out with a few headings, graphics and a bit of white space. Your reader will react the same way.
Do your headings catch attention? How much of the story do they tell on their own? Do they accurately represent the text following them?
How can you write good headings? Make sure that even a reader who skims your document takes away your message.