If this is a duplicate, it's one that a lot of people could benefit from.

Summary:

1. Delete redundancies. 2. Use numbers and specifics instead of adverbs and adjectives. 3. Add missing context. 4. Focus on the strongest argument. 5. Delete off-topic material. 6. Seek out equivocation and remove it. 7. Kill your favorites. 8. Delete anything written in the heat of emotion. 9. Shorten. 10. Give it a day.