Inexperienced writers often think of revision as a tidying-up activity which takes place after the real work of writing an essay has been done. In fact, revision is one of the integral phases of the writing process, and can make a great deal of difference to the mark an essay ends up with. Depending on your way of working, you may choose to revise earlier parts of the essay before you have got all the way to the end, especially if you are writing it on a computer.
If you spot mistakes or difficulties, it is important not just to remedy them in the places where they occur, but to ask yourself whether they are symptomatic of wider problems in the essay, such as writing which is writer-orientated rather than reader-orientated.
You also need to think about your essay not just as a self-contained whole, but in relationship to your goals and to the rhetorical situation in which your essay is placed. It is very easy to get so wrapped up in your own ideas that you allow the essay to drift away from the question and its associated expectations by not signposting sufficiently, or providing enough cues for your reader.