Do frame the nature of the philosophical problem clearly. For example, "Anyone claiming to know about a certain kind of thing ought to be able to say what kind of thing it is. . . ."
Don't go off on a tangent.
Do stick to the issues mentioned in the paper assignment.
Don't throw out opinions casually. Example. "Anybody can see that Socrates was being unfair to Euthyphro. I don't blame him for walking out; I would have done the same thing. In fact, I think philosophy is just a big scam."
Do give reasons for any opinion you express. "I find Plato's solution to the problem of what makes a thing what it is to be inadequate. To say that piety itself is what makes a pious act pious does not really answer the question. It invites the further question, what makes piety itself pious? Is it yet another form? I do not see how such a question could be answered finally." (By the way, Plato was aware of this argument, and Aristotle discussed it on pp. 113-114 of the text.)
Don't make undocumented claims about what any of the authors wrote.
Do back up your description of the philosopher's position by use of quotations from the text. Parenthetical page references to the text are sufficient in lieu of footnotes. For example, "According to Plato, the ability to state what makes something the kind of thing it is has practical consequences. Socrates insisted on this point when he said to Euthyphro, 'If you had no clear knowledge of piety and impiety you would never have ventured to prosecute your old father for murder on behalf of a servant.' (p. 28)."
Don't use the words of others without quotation. One good way to recognize when you are plagiarizing is to notice any change of style, say some sentences which use a lot of words you do not use ordinarily, or whose grammatical structure is very different from your own. (I use this test on my wife's essays!)
Do use your own words to paraphrase what an author says.
Don't neglect to address all points in the paper topic in detail.
Do provide sufficient detail on all points, so that the grader can recognize your mastery of them.
Don't pad your paper or eliminate vital parts to get it to the suggested length.
Do write economically. Make the paper just long enough to complete the required tasks and no longer. If you deviate significantly from the suggested length, consider whether you have said too much or left something out.
Don't simply write down a bunch of logically unconnected statements or assertions. For example, "Euthyphro said that piety is doing what he is doing. Socrates said he was wrong. Euthyphro agreed, and he tried again, saying that piety is what is loved by the gods. The second account wasn't any good either, so he tried a few more. Finally, he gave up."
Do present the material in the form of arguments. One way of looking at an argument is as the defense of a conclusion by appeal to premises which are uncontroversial. In the Euthyphro, Plato uses arguments of this form: If your understanding of the nature of piety is correct, then it has such-and-such consequences. But these consequences are unacceptable. So piety is not what you said it is. Euthyphro agrees to the two premises (i.e., about what follows from Euthyphro's account and about the unacceptability of the consequences), so he is forced to accept the conclusion. One criticizes an argument by showing that the conclusion does follow from the premises, or by disputing one of the premises themselves. For example, "Socrates demanded that Euthyphro tell him what makes something pious. If he could not do this, then he does not know about piety. But Euthyphro was able to recognize a common characteristic of pious acts, such as being loved by all the gods, so he could at least justifiably claim to know which acts are pious."
Don't use colloquial language to make a point. For example, "Euthyphro's first definition of piety was totally lame."
Do use standard language.
Don't confuse technical language with ordinary language. For example, Plato's use of 'form' is different from ordinary use, in which it means 'shape.'
Do explain any technical terms when you introduce them. For example, "The ontological argument is an attempt to prove that God exists simply from the definition or idea of God."