It's essential that before starting to type an essay, you've done all the essential reading. There is no point in trying to write something if you have no idea about the subject. When reading through materials, scribble down thoughts and ideas of how the content relates to the essay question. As you read more and gain a better understanding, group ideas to form arguments that you can use. Keep a note of where you read about key points so you can refer to them when you need to. Also read other essays covering the same subject to discover what works well.
Essays need to be logical and easy to read. They have to guide the reader through the information in a sensible way. Before writing, work out approximately how many words you'll write in each section. The structure depends on the total length of the essay but needs to include an introduction, your arguments and a conclusion.
You need to get the basics right. Although you learned grammar and spelling when you were a kid, it's amazing how easy it is to make mistakes when using a computer. Spell check can't tell when you accidentally type a word that is spelled correctly in a sentence. Always get somebody else to read through your work. When you're working on something for a long time, you often don't see the silly mistakes.
Allow plenty of time to complete all the necessary steps involving an essay: research, sorting out a structure, writing it, getting somebody else to read it, editing it and referencing. There is no point in spending too much time on one area, and you don't want to rush yourself unnecessarily.
Continually save your work to avoid losing it and having to start again. Don't rely on your computer not having problems, so always save it in a couple of places. This can be on a USB stick or by sending a copy in email to yourself.
Always reference your work properly. With lots of people writing the same essay, you need to avoid any chance of being told you've cheated. Use either footnotes or the Harvard referencing style so professors can check up on any work they're unfamiliar with.