In writing your history of philosophy paper, you may be tempted to pull a lot of quotes from primary (the author's work) and secondary (the commentators' works) materials. The best thing to do is to use short quotes, if you must quote, and to quote infrequently. Quotes from the primary material should be used to illustrate a point you are making. Quotes from secondary material should be used only if you cannot paraphrase what is being said. The most important thing to remember when quoting and paraphrasing other authors is to cite the text. Your professor will most likely have a preferred method of text citation - usually APA or MLA format.
Be sure to be concise and clear when writing your paper. Also, don't include biographical information about the author unless it directly pertains to the theory (very rare). When writing history of philosophy papers, you often will come across foreign terms. Foreign terms should be italicized in your paper. If you are using a translation, and the paper is an analysis of one of the terms used by the author, you may want to check the original to make sure that terms that appear the same in English are really the same in the original. As an undergraduate, this last point will often come from secondary material research, but as a graduate student you really should do the dirty work and look at the original.
Finally, when editing your paper, double check all citations, quotes, and paraphrases to ensure that you've properly attributed any knowledge that was not your own going into the paper. Check for spelling, grammar, and typos. Finally, check for accuracy in term usage. Add your bibliography and turn it in!