There are lots of things but there are few basic things which should be followed at all cost:
(1). Simple language. Don’t show off your vocabulary. Simpler the language, better it is. Examiner should be able to understand the essay in one quick reading.
(2). A fine balance between facts/phrase/quotes/logic
(3). Proper flow in entire essay. Interlink ending of paragraph with next para.
(4). Good introduction and conclusion. Introduction creates first impression. It should generate interest in examiners mind. It should also give a glimpse about your views and what is going to come in the body. Conclusion should be positive and visionary.