Greg Mankiw's Blog: How to Write Well

Here's some of the good tips from a list on writing well passed on by Greg Mankiw's Blog: How to Write Well

• Stay focused. Remember the take-away points you want the reader to remember. If some material is irrelevant to these points, it should probably be cut.
• Keep sentences short. Short words are better than long words. Monosyllabic words are best.
• The passive voice is avoided by good writers.
• Positive statements are more persuasive than normative statements.
• Use adverbs sparingly.
• Avoid jargon. Any word you don’t read regularly in a newspaper is suspect.

And of course, the last rule: keep it simple.