Assessing the coherence of your writing isn't just a matter of knowing there's an overall argument connecting the things you say, but of working out how easy it would be for a reader to reconstruct the mental picture which has guided your writing. At the level of the overall essay, the reader's sense of coherence can be improved by providing explicit information about your goals in the introduction, and by later signposting.
During the revision phase, it is also important to scrutinize your writing for coherence at the level of the sentence and the paragraph. Academic style requires sentences to be grammatically complete, as this is an aid to coherence, so that starting a sentence with a participle, or the words "and" or "but" is frowned upon. Excessively long sentences also threaten the reader's sense of coherence, as it is hard to keep such a sentence in your head as a whole. Within sentences, you need to make sure that clauses are given order through grammatical subordination, and not simply strung together by using "and".
Similar issues arise with paragraphs, which can be seen as forming units in the same way as sentences, although here the question of coherence is logical rather than grammatical. A paragraph should not just be an arbitrary break in your text, and this makes the beginnings of paragraphs particularly worth paying attention to, as it is here that you can establish the reader's sense of a coherent transition from one paragraph to another by making the content of the first sentence refer back to the preceding paragraph in some say (perhaps by repetition of a key word or phrase). Simply starting the paragraph with one of the connecting words, such as "however" or "therefore", does not help your reader's sense of coherence very much, as it leaves too many possible inferences open.