My tips, from someone who has written a fair share of well-received papers:
1.) Use spell checker, and keep a dictionary and general writing / grammar guide handy.
2.) Avoid long lists; they are almost always what one of my profs called "literary poison".
3.) Avoid cliches like the plague.
4.) Unless absolutely necessary, do not use encyclopedias; if you do need them, use only the bare minimum, and clearly reference it. Lots of profs see encyclopedia use as intellectual laziness.
5.) Write an outline, preferably with your topics organized thematically.
6.) Never neglect a good introduction and conclusion. It's best to write these two components last, rather than first when they'll only limit what you can do.
7.) For research papers, I find that I always gather more research than I actually need. Not only does this give you a safety net, it can force you to learn how to condense a lot of info into a concise statement; teachers love clear, concise writing.
8.) Footnotes or endnotes. They are a bitch, but teachers love them, and they show intellectual honesty.
9.) For long papers, make careful, judicious use of quotes. Too few may make the paper look bare and paraphrased, and too many will clutter the paper and look pedantic.
10.) Keep your notes handy--I mean literally at your desktop, for easy reference. This is where the good old spiral and marginal notes will be your best friends.
11.) Use lots of different sources related to your topic, and tie them together in your paper. It shows the degree of effort you're willing to make to prove a point or a theme, and in any event strengthens a paper, in my op. Relying too heavily on one or two sources never looks good, though I admit that sometimes it's almost inevitable. Try to avoid it, though.
12.) Start *early*. I'm a terrible procrastinator, myself, but I think that starting early on my papers helped me immeasurably; it gave me more time to polish my work, and reduced the cramming pressure that can muddy your focus and your paper.
13.) Use other good essays, or other nonfiction writing as starting examples. For instance, I was inspired by Peter Brown's "The World of Late Antiquity"; it condenses a huge amount of information in an incredibly short, concise book with a theme that is immediately present at the end. When I wrote my seminar, I went for the same effect, and my prof loved it.
14.) Write with the *intention* of making it look good, by taking your time and having a critical eye. This latter quality is especially important; the paper that you think is Pulitzer material today may look like a mad lib tomorrow. If there's one thing that profs hate aside from generally bad editing, it's papers that look like somebody cooked them up 10 minutes before class.
