Write in clear, concise English. Use the least number of words possible to make your point. The events have occurred already and should be treated as such. Do not use colloquial or abbreviated English. Complex points of debate or material which is necessary for background but somewhat tangential to your thesis can often be treated in footnotes, so as not to interfere with your main argument.
Short sentences are often easier to control.
This helps you to make your points clearly and forcefully. Frequent paragraph divisions may also help to maintain interest and to separate thoughts from each other. How you handle sentence and paragraph divisions is naturally a matter of taste. But keeping things short will usually at least ensure that your points come over clearly, your first responsibility. You can go after elegance at a later stage.