Research the previous literature on your topic long before writing the paper. For example, if your paper is about the postmodern dimensions of modern feminism, then you probably want to research previous books written about postmodernism and previous books written about modern feminism. These previous books are treasure troves of insight. Spend significant time (e.g. days or even weeks) reflecting on the content of this research before writing your paper. This will help add to the depth, complexity, and overall quality of the paper you write.
Outline the core elements of your paper. For a college philosophy paper, writing an outline is necessary, because it helps ensure that each of your ideas logically follow one another. For philosophy especially, if your sentences and thought-process appears illogical to the professor, then the paper is likely to get knocked down a grade.
Review your ideas with your professor before writing. She will help you hash through your ideas, most likely revealing the strengths and weaknesses of your argument and whether the overall paper is likely to earn a good grade. Review and edit your outline after the meeting.
Write the paper and stick close to all of the basic grammar rules. Given the frequent complexity and depth of most sentences in a philosophy paper, it is absolutely essential that each and every sentence is clear, concise, and logically follows the sentences that precede it. Philosophy papers frequently follow a specific logic from beginning to end, and a single unclear sentence can throw off the reader. Moreover, what the beaker is to the scientist, the word is to the philosopher. Words are the key instruments to express a philosopher's thoughts. Thus, the perfect expression of those thoughts is critically important.
Review the finished product numerous times before handing it in. Since philosophy should technically be understandable to anyone, try allowing a well-educated friend or fellow philosophy student to read the paper. Take special note of their criticisms and edit the paper one final time as you feel is appropriate.