First, make sure you know your content well. If you are writing about a book, or comparing two or more books or poems, make sure you read them (or re-read them) before you start writing so they are both fresh in your mind.
Next, take the question and highlight the words which are significant. For example if the questions was : In Othello, to what extent does his final speech affect our assessment of him? the key words to highlight would be 'to what extent' and 'affect'.
The next thing is structure: it is always easier to write when you have a solid- but not too detailed- essay plan. Put your groups of ideas into coherent paragraphs which build to your final argument. If you are arguing a particular way, it is sometimes best to start with a paragraph explaining the alternative direction, and then countering this with your stronger opposing arguments.
Remember! It's not always best to argue For, Against, For, Against. The best essays are the ones which make one main point in each paragraph, and link the paragraphs naturally to come to the final conclusion.
In your conclusion, make sure you DO NOT INCLUDE ANY NEW INFORMATION. Sometimes it is tempting to end with a flourish, such as an intelligent sounding quote, but the key is to keep everything very relevant to your argument throughout the essay. Draw together the key point you have mentioned in your previous argument and draw a conclusion from them. Make sure you do not repeat yourself, and when you have finished, reread the question and ask yourself whether you have answered it.