1. Have something to say.
If you don't have something to say about the topic on which you are writing, you need to read the source again, do research, discuss with peers, and draft until you do. Clichés and recycled ideas are boring. Assertions that are not supported are suspect. Know what you are talking about and never assume that the reader will not know more than you. We are drawn to what is unique; find an angle and develop it.
2. Be simple. Be honest. Be clear.
Empty introductory sentences are unnecessary; they interrupt the flow of thought and can be very frustrating for the reader. Long convoluted sentences diffuse your ideas; the thread of a thought is easily lost. State what you mean to say directly and in an active voice. There should be no unnecessary words and no unnecessary thoughts to distract the reader from the point you are making. Make every word tell.
3. Every sentence should contain information that advances your story.
Sentences that contain no useful or compelling information are a waste of the reader's time. Adjectives should be precise and relate directly to the subject. Vague descriptions only weaken your writing, while specificity is always interesting. Do not overburden your sentences with detail; give enough salient information to clearly illustrate your point and no more.
4. Structure should carry the story.
Writing should flow from one piece of information to the next in such a way that the reader is not confused and does not have to double back or read ahead to know what is going on. Assertions should be made and details given that support an eventual conclusion; this is true of sentences, paragraphs, topic discussions, and the entire essay. A poorly structured essay can be confusing to the reader and will weaken the writer's argument.
5. Support your assertions.
A direct reference or pointed quote can illustrate what a paragraph of generalities never will. Choose your citations carefully and be sure they relate directly to your point. Do not overload your work with quotations; use them only as needed to support your assertions. Always cite others' ideas bibliographically in the correct format specified, (in my case, MLA). From now on, you can't just say stuff; you need to back it up.
6. Proofread your work.
There is little that is more frustrating to a reader than writing that is riddled with misspellings, awkward constructions, poor grammar, and logical dead ends. Read your paper carefully and use a dictionary, a thesaurus, and a grammar guide. Leave time to let the paper rest before re-reading it. Try reading it aloud or having someone else read it. If it's confusing to read aloud, it's confusing to read.
Richard's Rules for Writing
(Still my favorite.)