To write a good philosophy essay, you need to start your essay early, make sure that it answers the question, and does so critically and with direction. You also need to provide some thoughts of your own on the topic, and be sure that what you've written is easily understood.
Write early
Good essay writing is good rewriting. One feature that a good essay needs is a clear overall structure, and that can't be achieved at first pass.
Write with focus
Answer the question set. Do not just tell us everything you remember on that particular topic. So if the title is "Evaluate one objection to the ontological argument", do not mention other arguments for the existence of God, more than one objection to the ontological argument, arguments against the existence of God, or anything else (unless you specifically and clearly explain why it's relevant to your answer to the main question). Relatedly, unless the question specifically asks you to, do not write anything on who was born when, where they lived, or how long philosophers have been interested in the question.
Write critically
Don't merely describe which possible views there are, and don’t merely describe the arguments for and against those views. Evaluate those positions and arguments. If Gaunilo has an objection to Anselm's argument, don’t just state it. Tell us whether or not it works, and why. Equally, remember that you need to justify your conclusion, not merely say that you believe it.
Write with direction
Again, you aren't simply describing the debate. You're trying to lead the reader into a position on it that you think is correct. So don't tell us that the problem is difficult and no-one has solved it yet, or that you aren’t sure of the answer. Pick a position that you think is least hard to defend, and defend it as though you were certain it were true. (A caveat: You can argue for a conditional conclusion: Anselm's argument works only if...)
Write independently
Try to offer some independent thoughts on the issue. Parroting back the content of the lectures or the textbook will not be enough for the higher marks.
Relatedly, use quotes sparingly. Quote only when you are such a huge fan of the literary merits of the relevant section of text that you don’t think you can express it any more clearly in your own words. Putting things in your own words is a great way to show you understand something, and can help you frame the issue in new and original ways. This all comes with the caveat that you should not plagiarise: meet this constraint by referencing the source even if you don't actually use their words.
Write transparently
You should know what each paragraph and section of your essay is doing, and why they follow on from each other in the order that they do. And the reader should know this too. You allow them to do this by signposting paragraphs with phrases like "One objection to this argument is...", or "In summary...".
You should also provide the reader with a brief description of the overall structure of the essay. This is an excellent way to start your paper. The first two lines of your paper might be: "In this paper, I will argue that ___. I will argue for this by showing that ___." This way, as soon as the paper begins, the reader already knows roughly how the essay is going to proceed, which makes it much easier to follow. You should also put something very similar in your conclusion: "In this paper, I have argued that ___. I argued for this by ___".
Write in English
Don't leave spelling or grammar mistakes. First, we mark you down for poor spelling and grammar. Second, philosophy is hard to follow at the best of times, and it's ten times harder to understand what you're saying if it's not even coherent English. So spellcheck and grammarcheck your work, and read it out aloud to a friend to pick up on things the grammarcheck misses. (If you're not sure if you're using a word correctly, google it.)