After you have written the first draft of the essay, you need to move on to stages of revision, since no writer, no matter how skilled, gets everything exactly right the first time - of course, it's impossible to revise effectively if you've left working on the essay until the night before the deadline. First, you need to review your essay to see if the form in which it has come out is really appropriate to its rhetorical situation - it often makes problems easier to spot if you leave your essay for a couple of days and then come back to it. A very common problem which often emerges at this stage is that the essay is not sufficiently reader-orientated - perhaps your thinking changed a bit while you were writing the essay, and your conclusion contains material which really should be in the introduction.
Having considered issues to do with overall structure, you then need to move on to a detailed editing of your writing, asking yourself the question of whether you have managed to express yourself appropriately. This does not mean making your writing conform to some preconceived ideas about academic style, but asking yourself whether your writing conveys clearly what you were trying to say, and changing it if necessary. It's often useful to get someone to read over your essay at this stage and point out what they don't find clear.
After all this revision, you need finally to look at your writing from the point of view of coherence. Does one paragraph follow obviously from another, or are there unexplained transitions between paragraphs, where, for example, you stop talking about one text and start talking about another without explaining what you are doing? Perhaps you need to put in additional cues for your reader, to explain the sequence of paragraphs. Do sentences within the paragraphs flow from each other, or are they choppy and unconnected? Adding connecting words or phrases might help your reader form a mental picture of your argument.