Odds are, in first or second year university, your exam or esssay will be marked by a teaching assistant.
The teaching assistant is probably marking from an answer key or rubric. He or she has a list of things that she is looking for. If you give the TA all of the things that are on the list, you will get an A. If you don't, you won't.
What is on the answer key varies from course to course, and what I'm saying is most relevant to economics students, but I hope some of it would be general enough to be of benefit to, say, someone writing an environmental ethics essay.
First of all, answer the question. Make it obvious that you're answering the question. Whoever is marking your assignment may be doing so at 3:00 in the morning when he's half asleep, or while lying on the sofa listening to The Colbert Report. So hit him over the head with the fact that you're answering the question.
Take, for example, this question:
True, false or uncertain: “Utilitarians believe that income should be distributed equally among all members of society”. Discuss.
A good answer will start out with a position, for example, "it is false that Utilitarians believe that income should be distributed equally", and defend it. Subtle? No. But after five hours of marking, the TA will miss subtle.
Second, tick off the easy stuff. Any question is designed to test understanding of basic concepts and analytic skills. A simple way to demonstrate that you understand what a term means is to explain what it means in your answer! So explain what a Utilitarian is. If you were in a philosophy course, you might say someting like (borrowing from Wikipedia here)
In an economics course, you might say something like:
A Utilitarian is a person who believes that the best course of action is one that maximizes total social welfare, where social welfare is the sum of each individual person's welfare, i.e.
Other easy ways to pick up points: figure out how to work in a few of the concepts and ideas used in class. For example, pick up some of the articles from the course reading list and work them in. If your prof discourages that, use google scholar to find papers that cite the articles on the reading list and work those ones in.
Third, demonstrate that you have learned to apply the tools and methods of analysis taught in the course. As an economist, I don't care whether or not your philosophy prof taught you that consequentialist ethics are fundamentally flawed. Likewise, your philosophy prof probably doesn't want to to be subjected to a discussion of the appropriate functional form for the social welfare function. Yes, of course I care about you as a human being, but when I'm marking all I'm interested in is whether or not you've actually retained and figured out how to apply what I talked about in my course.
The actual answer is less important than the methods and analytic strategies, used. On an economics exam, that's easy (for me). Draw a diagram. The essence of success in undergraduate-level economics courses is figuring which diagram to use for which question, understanding what each curve in that diagram represents, figuring out how and why the curves shift, and learning how to explain and interpret the results. (Classic studying strategy: take every diagram in the textbook where a curve moves in one direction. Odds are, on the exam, there will be a question where the curve moves the other way. E.g. if the textbook shows income and substitution effects associated with a price increase, the exam will probably have a question on the income and substitution effects associated with a price decrease).
Essays are far more difficult. Every one is unique, and there are no hard and fast rules. The only general piece of advice I can give is: make sure that the logical structure of your argument is blindingly obvious. Remember, the TA is marking your essay in a hurry because she herself has an essay due this Friday. If the structure isn't obvious, she won't see it. The best indicator of a well-structured paper is that the paper is clearly organized into sections (either implicitly or explicitly) and paragraphs. Honestly, when I see an essay doesn't have clear, well-structured paragraphs, my immediate reaction is "unlikely to be an A paper." From that point, you have to work pretty hard to convince me otherwise.
What if you don't know what the logical argument of your paper is? Try using a flow-chart, or bubble diagram. Take all of the paragraphs in your paper, and write down in one sentence the point being made in that paragraph. (If you haven't written your paper yet, take write down all of the points that you want to make, all of the issues you want to discuss). Perhaps put each point onto a separate piece of paper so you can move them around. Can you put the points in any kind of logical order, e.g. A implies B implies C? Can you group them into categories, e.g. these are arguments for income redistribution, these are arguments against? Try to put the points into groups of three or four, and then figure out how these groups of three or four relate to each other.
So - get the paragraphs right, and the essay will take care of itself. One idea per paragraph. One paragraph following logically upon the other. With the question about utilitarianism and income distribution, I'd begin, as noted earlier, by defining all terms. I would then have a paragraph discussing the relationship between income and the thing we actually care about, people's well-being. A crucial argument to make is that well-being is increasing in income, but at a decreasing rate. People who are poor value an extra dollar's worth of income more than people who are rich. I'd then have a new paragraph, arguing that, given this fact, it is potentially possible to increase overall well-being through income redistribution - taking money from people who value it less on the margin, and giving it to people who value it more. I would then devote some paragraphs to possible complications: what about the costs of income redistribution? Differences in needs? Alternative conceptions of justice, for example, those based on notions of human rights? Etc.
But all of that wouldn't matter all that much. Whoever is marking your essay will probably decide in the first few paragraphs whether she likes it or not. If you can get those right, the rest will follow.
Last tip: page numbering shows that you care. As does stapling your assignment together.