The essay has to have a structure into which each paragraph clearly fits. Otherwise, and particularly if the paragraphs contain bits that don't cohere, the marker feels like being on a roller-coaster ride. Only somewhat less stimulating, if you see what I mean.
So the golden rule is: at every point in the essay, the marker should be aware WITHOUT EFFORT of where s/he is in the overall structure of the essay. Now, this means that you have to do a certain amount of signposting, even when it seems obvious to you what's going on, so that signposts seems like overdoing it: remember that while the essay might be your precious life blood poured out with pain and immense effort on the page, for the marker it's just words, and it's hard to concentrate on just words for a long time. I don't know anybody who actually enjoys marking essays, however good some of them may be. Would you? So don't make it hard, or you risk irritating the marker: they're likely to be pretty irritable to start with.
This means that there has to be a clear structure in the first place. A PLAN. Not just a general idea of where would be a good direction to go, so that the essay sets out hopefully but vaguely in that direction, rather like Don Quixote, and relies on luck and inspiration to get itself somewhere near where it would want to go if it knew where that was. No. A professionally produced piece of work is built up step by step, block by block: each block fits to the last and the next and has a function in the whole. If not, the house falls down.
To this students say (sometimes): "I can't write like that." When cross-questioned about why not, they usually say something about inspiration. Well, I don't mind how you get your inspiration, and if you can only get it in the act of writing, that's fine; I work that way too, sometimes. If the ideas come out in a stream-of-consciousness gush, then so be it; but I (speaking now as a marker) don't want to see it. I want to see the final version, not a first draft. After you've got your ideas, however you get them, you should put them in proper order so that they can be communicated.
The way to do this is to think in terms of headings and subheadings. For an example of this, see my Freud lecture. I've prefaced it with an outline of the entire lecture. If you adopt a hierarchical numbering method, as I've done there (1, 1.1, 1.1.1) for each heading it will discipline you into coherence and structural thinking. If you start by constructing a set of headings and subheadings, when you've done that, you can write each subheading up into a paragraph, and the essay will flow properly and be properly comprehensible. Of course, you should remove the numbers at this point: the convention is that literary essays are not sectionalised or numbered.
If you do this structuring carefully, eventually you won't have to do it consciously at all: it will come naturally, and your thoughts will automatically order themselves. After years of thinking this way I find that when I have to hold forth to a class on some subject or other, say in answer to a question, or when I'm being asked questions in court about handwriting, it tends to come out in neat (invisibly) subheaded sections, so that everyone (or so I fondly hope) knows exactly what I'm talking about. Well, some of the time, anyway. It's not actually that difficult: it's just a habit of thought. It's very valuable. If you have any doubt of that, just compare any lecture you've heard where there is this clear structure with one in which there isn't. No contest, right? So, if it's valuable, why not do it?
There's a useful article on this in The Oxford Guide to Writing.