What essay markers look for
The key thing to demonstrate in the essay is that you understand the subject matter, be it Marx's ideas on class society or the sexual habits of leading social theorists. To do this, show clarity of argument; understanding of the reading used; and if possible some original thinking. There is nothing mystical to it: these are all skills that come through practice. Ensure you get the maximum amount of feedback on your essay so you can develop your style to include these things.

What essay markers don't like
Over-reliance on the core text is annoying, as is material clearly paraphrased from it. This makes for boring reading, and one thing you don't want to do is bore your marker. Also, don't use terms or arguments you don't really understand. Markers can usually see through this, and it just irritates them (they're a sensitive bunch). Limited reading and derivative arguments are bad, as is a weak conclusion which has little to do with the essay question. Rambling off the essay topic and including irrelevant material will make you look like you don't really understand either the question or your own arguments.

Selecting an essay question
Some courses offer more choice, some less so. Whatever the case, it is important that you make the most of what choice you have. The question that interests you most, which is often the one you will have the strongest opinions on, is usually the best to choose for an essay subject. This'll make the whole process more pleasant, and allow you to put more of yourself into the work.

Writing a plan
Structure is vital, and get it down right away. It is best to write some sort of a structure before you start the reading, as this will allow you to slot the material you get from the reading into the structure.

Strength of argument.
You have to be both balanced, in summarising the various viewpoints on an issue, and opinionated. The whole essay should work towards a conclusion you are happy with, and the stronger the conclusion the better.

Reading
Both what you read and how you read are important. Selecting reading is often difficult - particularly if the only copy of the key text for your essay topic seems to be permanently out of the library. It is often hard to tell from a list of titles what is relevant. The first stop if you are unsure should be your tutor, who will be able to point you in the direction of something intelligible. It is vital not to rely on the central course textbook for an essay - a reworking of the one thing you can be sure your tutor has read is going to ensure you are marked down as derivative and unoriginal. When taking notes only write on one side of the paper.

How to read is as important as the choice of texts you make. With limited time, and the delights of Neighbours a continual temptation, you want to make the best of your time. If reading a book, select the important chapters, according to the structure you have already written. Articles often benefit from being read more than once, this being easier as they are shorter. At this point the benefit of having prepared a structure in advance will become clear to you, as instead of taking swathes of notes you can slot each note you take into its place in the essay structure. This should save time and effort, and ensure that everything you are putting into the essay is relevant.

What to put in
Restraint is important. Don't take a kitchen sink approach, and assume that what your tutor wants is for you to throw in everything you know about the subject, or worse, everything that is in the course text. This will just make it look like you have rehashed the course textbook, a guarantee of a mediocre mark.

Original material is the clincher that makes a good grade essay into an excellent grade essay. There are several sources to draw on for originality. Develop your own argument on the essay topic which contrasts with that contained in the lectures or the course text. Include personal experiences where relevant to the topic. Use work from other subjects to give an unusual or complementary perspective on the work under review. Philosophy, history and psychology can often give an unusual and interesting slant to a sociology essay, though don't overdo it.

Writing style
Academics fall prey to a number of irritating habits. These include the use of the passive voice and overlong sentences. The passive voice is something like, 'The evidence suggests that...' 'The research indicates...'. Well, who suggests, who indicates? Though often unavoidable, the use of the passive voice should be kept to a minimum. Overlong and convoluted sentences are murderous. Ideally, try and keep them to around a dozen words long. It is easier to read a set of short sentences than a few very long sentences with too much information in each to be taken in properly.

Avoiding being derivative or plagiaristic
Plagiarism is defined as direct copying of another's work and passing it off as one's own. It will ensure your essay is returned unmarked. If persistent, severe trouble may ensue in the form of a rapid expulsion, so don't bother. Everything must be in your own words, unless in quotation marks.

Being derivative is easy to do, and is a much bigger problem than plagiarism, especially with that old 'A' Level essay lying about the house and temptingly similar to what your essay will be on. Don't use overlong quotations - try and put everything in your own words. This is a matter of practice, and you will soon be able to word arguments in your own personal style.

Referencing
The University Library publishes a guide to the standard Harvard referencing system, but here's a summary. Reference any work whose argument you are using in your essay. If that work is itself summarising another, then you can either reference it or the work being summarised. Do use the references/bibliography to show your tutor that you have gone beyond the course text. References should be indicated by the use of the author's surname and date of publication in brackets, as in (Russell, 1984). Then in the bibliography list it thus (the system varies somewhat) -
For books:
Russell, B. In Praise of Idleness, London: Unwin Press 1984. (author, title, city of publication, then publisher)
For articles:
Hawes, D. "Gypsy Site Policy - A Failure of Both Carrot and Stick." Policy and Politics 15, 1, 49-54. (author, article title, journal title, volume, issue, page numbers)
For book chapters:
Weber, M. "Major features of world religions", in Robertson, R. (ed) Sociology of Religion. Harmondsworth, Penguin, pp 19-41. (author, chapter title, editor, book title, city of publication, publisher, page numbers)
This is rather a lot to remember, but you'll get used to it.

Grammar, spelling and presentation
It's amazing how annoying this can be; correct punctuation and spelling make a huge difference to one's essay, and simple grammatical mistakes detract from an otherwise fine piece of work. You should as standard word-process your essay. Your tutor will be eternally grateful not having to decipher your appalling handwriting, though this will not always be reciprocated and you will while away many happy hours of your University career deciphering the kind of scribble that looks as if it was made by an acid casualty on a particularly bad flashback. I have never understood why academics feel they have to write in the kind of simian scrawl that would in the Secret Services be mistaken for a fiendishly unbreakable code. Nevertheless, word-processing your essay allows you to use the spell-checker, thus gaining valuable brownie points.

Wrapping up
The conclusion of your essay is when you should bring together the strands of the arguments you have been developing, and give something like a direct response to the essay question. It doesn't have to be just one paragraph. A good guide is to have one or two paragraphs summarising the strengths and weaknesses of the arguments that you've put in the essay. Then have one paragraph on your own opinion, which should ideally point to some weakness in the essay question, or attempt to go beyond it in some way.

And finally
Always re-read what you've written. Even better, get a friend to read it - preferably one who will tell you if you aren't making sense. Better they should tell you than you find out from your tutor.