The single most important feature of your prose is its clarity. Since you're writing a philosophy paper, you might feel that you must wax profound and use complicated sentences with lots of subordination (as the authors you’re reading often do!). Resist this temptation! Aim for simple, clear, and precise prose. (What's clear is relative to your writing skills.)
Work hard to say exactly what you mean. If you're unsure of the word you want, look it up: figure out precisely which word makes your point.
Make sure that your thoughts follow a well-organized pattern. Don't skip from idea to idea or wander from point to point. Plan your paper to have one central thesis, and make every part of the paper serve the argument for this thesis.
Write an introduction and a conclusion. Your introduction should, well, introduce the reader to the themes and issues you're discussing, as well as state compactly the thesis you're going to defend. Philosophy papers are not mystery stories: let the reader know your conclusion up front. The concluding paragraph should at least restate the conclusion of the paper, as well as, perhaps, point to unresovled issues, remind the reader of assumptions for which you have not argued in the paper itself, and/or remind the reader of the train of your argument.
Do not present the autobiography of your paper writing experience. Don't write that first you thought x, and then you thought y, etc. Discuss the material.
When you present an argument of your own, or an objection to someone else's argument, be sure to consider responses to your argument or objection. One trick is to imagine that you're a philosophical lawyer and that you have to convince a (rational) jury of your case. Every time you present an argument or objection, the other lawyer will have a chance to respond. So, you must anticipate objections and come up with responses to them. In your paper, I want to see you actually do this: say how a critic might respond to what you're saying, and then defend yourself against the response.
How many objections should you consider? A general rule to observe is that exploring fewer and more challenging objections is better than more but less challenging objections. Partly the answer lies in how long a paper you're writing.
If the assignment is for a five page paper (as it almost always is in my introductory courses, Phil. 20 and 98), then use only one objection. That is all you have time for in a five page paper.
For my other courses, in which you're writing longer papers: the longer the paper, the more objections you can follow out. Note, however, that it is often more useful to consider an objection to a response than to open up an entirely new line of objection.
One of the risks of considering too many objections is engaging in "hit-and-run" objecting. What I mean is this: any objection that is worthy of your time in a formal philosophy paper should be substantial enough that it teaks a good meaty paragraph to present, and another good meaty paragraph to rebut. (In a longer paper, objections can take a couple of pages to develop!) So, if your objection (or response) is a single sentence or just a couple of lines, it's almost certainly too thin.
How does one respond to an objection? A response to an objection is most often, in effect, an objection to an objection.
Do not quote too much. Too much quotation is not only boring, but takes up valuable space that should be used for the development of your own explanations and criticisms.
When you attribute a claim to an author, be sure to back up your attribution with a reference. I have no specific requirements concerning citation style. Whatever you do, use some consistent system of citation.
Be sure to be meticulous about giving credit to sources, whether they be books or discussion partners. Note that plagiarism is a violation of the Honor Code. Make sure you know what it is, so that you don't do it by accident!
As much of the intellectual world has moved online from printed journals and books to webpages, blogs, e-books, and journals published online, it has become increasingly difficult for untutored eyes to distinguish between credible, professionally curated, peer-reviewed intellectual material and the most recent fever-dream of a crackpot. If you go online to do research, be sure to verify that the sources you are using are intellectually credible. This can be hard for novices, and so in my introductory courses, I generally recommend that students do no philosophical research online. (The assignments also generally do not require or invite research anyhow.) The Web can be a useful source of factual information, but be sure that you may be confident in the reliability of the specific source you use.
Do not rely on dictionary definitions of terms that are "in play" in your discussion. For example, do not turn to a dictionary to settle the definition of "free" or "responsible" or "real."
Make sure that the basics are in order: don't misspell words, don't use incomplete sentences, and so on. Especially annoying are apostrophe errors, and they will be treated harshly.
See my Notes on Style and Grammar.
Leave out biographical and historical data about the authors whom you discuss, unless that information is relevant in explaining how an issue to which an author was responding has changed since they wrote.
Leave out broad brush stroke intellectual history. Don't tell me that some problem is "perennial," or that philosophers have long questioned whether p.
The paper assignments in this class do not ask you to express your feelings about the topics, nor do they ask for your "personal reflections" on the issues. They call upon you to reason and to argue for some position, in either agreement or disagreement with an author. This requires you both to interpret the author plausibly and to provide cogent considerations for your positions.
Don't go to the library or online to get help. Lots of the books in the library and many websites are not very good and will fill you with misinformation. And in any case, I want to see your thoughts, not someone else's. Your ability to generate ideas from your own resources is part of what I will be evaluating in your paper.
Do feel free to consult with your classmates, however. Since philosophy requires that you engage in dialog with other views, it makes good sense to run your ideas up against a classmate. The paper must in the end, nonetheless, be yours. So, if you get an idea from a classmate, give him or her credit in a footnote.