Writing an essay requires lot of homework. Firstly it also depends on the type of essay or the premise of the essay. In case it is an extempore essay then one naturally has to have a good command on English and also a clean thought process. As you receive the topic, it is better to do an analysis of your knowledge level. For any kind of writing, reading is important. A well read person can enhance his vocabulary is exposed to various styles of writing. Reading a novel, a magazine or a newspaper introduces us to print literature and also each writing is different.
Human mind is free flow in thought. Supposing you are a master at the topic of the essay then writing will be easy. It is fine to deviate a bit but always maintain a focus in the ending lines of each paragraph. Indexing, punctuations and number of words have to be maintained as per the requirement. In case it is a casual essay, which has to read in the form of a speech or comment, it is fine to use a bit of humor or fitting anecdotes.
Using similes and phrases in essays makes it more interesting and helps us to draw more attention to the sequence of events. If possible and if time permits, it is better to jot down points or words in a rough column so that there is more focus. The most important thing about an essay is to stay calm and not finish it in a hurried manner. If at the end of the essay you have a novel thought that will add spark to the article then you may delete the lines which are rambling.
Attempt with a fresh mind and in case the topic is novel then recollect a few points and then concentrate in tuning the article. Many times the mind gets jumbled and we do not know which point needs to be highlighted. In such a case, either you could write a lengthy article or concentrate on editing it or else choose sub points and highlight specific points.
Editing is important to check for grammar, syntax, lingo and tone of the essay. The essay should be general and not preaching. There has to be one point which is of prime importance and a good writer usually cashes in on that particular aspect.