Too many people, at least implicitly, believe that words are disposable and interchangeable vehicles for ideas. This is a mistaken view of the relationship between words and ideas.
Your words are not the vehicles for your ideas – they are your ideas.
To quote the philosopher John Searle, 'If you can't say it clearly, you don't understand it yourself.' In other words, a confusingly worded argument is not the result of someone with a clear understanding of an argument choosing the wrong words. Rather, it is the result of someone lacking a clear understanding of the argument – the confusion in the mind produces confusion on the page. Confused ideas and confused words are the same things; using language well and thinking well are the same thing.
Being clear and being simplistic are not the same thing. The best arguments, when put forward by the best philosophers, are often both subtle and clear; both intellectually challenging and lucid. Do not think that a clear and transparent argument is a simplistic argument; it is a well-expressed argument. Aspire to clarity, not to obscurity. As Thomas Huxley put it: 'Obscurity is more often the result of the muddiness rather than of the depth of the stream of thought.'
Since essays are about showing that you have understood ideas, it is essential that the words/ideas you choose are the right ones. After each word, each sentence, each paragraph you should ask yourself, 'Is that exactly what I meant? Do those words together form a clear and coherent idea? Or has my choice of words produced an ambiguous, wordy, obscure or clumsy sentence?'
Do not reproduce the phrases and constructions used by the authors you are discussing when giving an account of their views (other than when you are quoting from them, of course). Mixing their phrases with your own almost always results in awkward and unnatural writing. Use your own words. Putting ideas into your own words ensures not only that you write more clearly but also that you have understood the point. More often than not, the reason we resort to using someone else's words is that we don't quite know how to put the point ourselves, because we don't really understand it. This is another example of how clear writing and clear thinking are two sides of the same coin.
Finally, be prepared to rewrite sentences. Writing on a word-processor makes it easier to do this. Your first attempt at a sentence will often be awkward or unclear and in need of revision or rephrasing.
Such perfectionism about word choice can be time-consuming initially, but it is the essential basis of an excellent writing (and hence thinking) technique. With time, good word choice becomes second nature.