Spread your writing over multiple days or weeks, this will allow you to do proper research and planning before your due date, you'll be able to write a rough draft before your final copy. Each time you sit down to write give yourself a word count goal to achieve, you'll accomplish more. When you wait until the night before your paper is rushed and you're not able to do a proper review and edit.

Go over the assignment and make sure you understand what is expected of you. Follow your teacher's requirements; is there an assigned topic? How long should your assignment be? Does it need to be in a specific format?

If you're unclear on any instructions ask your teacher far enough in advance of your due date.