The following are just reminders of the most important strategies for the task. For more detailed discussion of principles of clear writing, I cannot recommend highly enough The Elements of Style , by William Strunk, Jr. and E. B. White. State your thesis clearly and support it well. As an argument must have a specific conclusion that follows from specific premises, an essay must have a specific thesis that is supported by specific evidence. It is best to make your thesis clear at the beginning of your paper, and to indicate there, in brief, the evidence that you will consider.
Evidence includes explanations, arguments, and definitions as well as facts.
Keep your priorities straight. Every essay is limited by considerations of space. Therefore good judgment is necessary about what to include. All information that is necessary for establishing the thesis must be included, and all irrelevant information must be excluded. Everything else will be more or less important, and your task is to judge which information is more important, and include as much of it as limitations of space allow.