1) Demonstrate your understanding 
The main aim of any philosophy essay is to demonstrate your understanding and mastery of the issue addressed by the question. Doing this generally has two components: exposition and critique. 
Exposition: You should expound the subject matter addressed by the question (in this course this is generally linked with a particular author). By doing this you demonstrate your knowledge and understanding of the positions under consideration and the question. For example, if the question asks you to assess Sartre’s idea of bad faith, you will need to make clear what Sartre understood bad faith to be and use your exposition as the basis for an evaluation of his views. 
Critique: As a philosopher you are expected to retain your intellectual independence and to judge critically the material you are discussing. ‘Critically’ does not mean that you necessarily reject views, but that you understand and make clear what counts for and against them. By critically examining other authors’ views you demonstrate both a higher level of understanding than in merely expounding them and your own intellectual autonomy. You should write in a way that makes clear when you are expounding and when you are criticizing the views of others, so that you cannot be interpreted as having yourself confused the two (see also point 5 below). The simplest – but not the only – way to do this is to keep exposition and critical discussion separate, e.g. to present Nietzsche’s views about eternal recurrence before going on to consider problems with or to defend those views. 
2) Argue your answer 
Set essays in philosophy invariably require you to develop a view of your own about the question asked. It is not enough just to state your opinion (‘Merleau-Ponty claims … I think he is wrong.’). Nor is it of particular relevance to say how you ‘feel’ about the ideas you are discussing. Rather you should aim to set out clearly which factors must be considered, which count for and against your views, and how you arrive (rationally) at the answer you are presenting. The aim of your discussion should be to make it hard for others to disagree with you, because you have considered all the important factors and come to the correct conclusion. Your reasoning will invariably be more convincing and more sophisticated if you anticipate and refute likely objections to the view you are arguing for. In writing your role model should be not an advertising executive or a political campaigner, whose main aim is to persuade people by all available means, but the judge in a court of law, who is supposed to weigh up all evidence, for and against, and come to the best decision possible. (Note that for grading purposes it is far less important what your conclusion is than how clearly and how well justified your views are. In particular, it is fine for you to defend a position differing from the lecturer’s – or the view you attribute to the lecturer. What matters is that you do a good job of presenting and defending your view.) 
3) Be selective 
You will not be able to discuss every possible aspect of a problem or issue, especially in a relatively short essay. Part of understanding a philosophical issue is to understand which considerations are most important and which are peripheral. Accordingly, in writing your essay it is a good idea to focus your discussion so as to develop a limited number of points in greater depth (through exposition and critique). For example: If you are writing an essay about Heidegger’s views on truth, it is better to discuss, say, two or three of the central features in some detail rather than trying to produce a long list of ‘bullet point’ characterizations. 
4) Organize your thoughts 
A good essay should have an overall organization in which the role of each part is clear. For this to be the case you should avoid redundant, irrelevant or disconnected sections (or even sentences) and structure your text so as to define a clear line of thought. As a test you might ask yourself whether someone who sees your text for the first time will understand what is going on in each part of the essay. 
Two more concrete suggestions: First, allow time to review and revise your essay rather than submitting your first full draft. It is surprising how many things seem less clear when returning to and re-reading a text with some distance, even if it is only a matter of 1-2 days after writing it. Second, the introduction-discussion- conclusion format might seem old-fashioned, but it is a well-proven way of ensuring that your main message cannot be overseen. Writing an introductory paragraph last, once you have already worked out your arguments, is a good way of making these clear to your reader (and often to yourself). Similarly, a brief concluding paragraph can help to bring the central results established by your essay into focus. 
5) Distinguish voices 
An important, but sometimes overlooked aspect of the clarity in writing is identifying who owns the various views under discussion. You should aim to make as clear as possible whether each claim is being presented in your (the author’s) name, in the name of some other author whose views you are considering, or impersonally (‘a possible objection to this is …’, ‘it might be said …’ etc.). This is important because it should be recognizable where you agree and where you differ from the views under discussion, i.e. it is an important means for demonstrating your intellectual autonomy.