So far, no one's talked about structure, which is one of the most important aspects of essay-writing. I've always found that I do better on essays if I stick to a formal structure (the "This is what I'm going to tell you, now I'm telling it to you, this is what I just told you" structure). This structure is as follows:

1: Introduction: "This is what I'm going to tell you." Your thesis sentence should be either the first or last sentence in this paragraph. The rest should be made up of a brief and generalized description of the aspects of whatever you're writing about that are important to your thesis. You should also (this is the most important part) state ALL your points in a brief (one sentence) and general form here.

2...n+1: Points: "Now I'm telling it to you." Always decide what points you're going to bring up to support your thesis BEFORE you start writing. I recommend that n = words / 500, where words is the number of words in your essay. n shouldn't be less than 2, and going over 5 or 6 points might be pushing it. Don't forget to start each of these paragraphs with a topic sentence and end it with something that leads into the next paragraph. Flow is important. Always use two examples if you can; three if you're trying to squeeze a few extra words into your essay, one if you can only find one (if you can't find any, your point isn't valid).

n+2: Conclusion: "This is what I just told you." Repeat your points once more, just as you did in the introduction; one sentence each (but I don't have to tell you not to use the SAME sentences as in your intro). Then state your thesis again, but in a final sort of way, as if you've proved something beyond any doubt. But don't use the words "in conclusion" here. Many instructors hate that.

One other important point about formal essay-writing: never use the first or second person! Writing only in the third person makes you sound intelligent (as well as satisfying old-school instructors who prefer very formal essays). Instead of saying "Therefore, I think ," write "From this, it is possible to conclude ," or "One can only assume that ." Definitely don't use "you." In fact, don't refer to your reader at all.