Then one should think about the topic and if it is one where one has to argue for or against the motion, then one should think about the topic before beginning to write the essay and figure out whether one agrees with the motion or disagrees with it. An essay should also have its arguments in a point wise format. This does not imply that the sentences should not be complete and should be bulleted but rather it implies that the essay should have certain well defined points. While thinking about what one is going to write, one may jot down in brief the points that come to one's mind and then use them as reference points while composing the essay.
Division of an essay into paragraphs helps to add a visual dimension of coherence and structure to the essay. If one has a few major points, then each point can be dealt with in separate paragraphs. An essay should also have an introduction and a conclusion. The introduction should clearly set out one's goal or argument, then one should make one's argument and finally, the conclusion should sum up the arguments that one has made.
Most essays have a word limit. Whether one is writing an academic essay or a college application essay or a scholarship essay, it is important to abide by this word limit as one must remember that the word limit is there for a purpose. If the essay is going to be published, the word limit serves to indicate the space limit. If the essay writer exceeds the word limit, then the editor has to go through the essay and delete words thereby making the essay different from what the original writer had intended. Similarly, if it is a scholarship essay or a college application essay, the word limit serves as an indicator of time. The readers have to go through several (in hundreds and thousands probably) essays of that kind and it is time consuming and also boring for them to go through a very long essay. They look for applicants who can make their arguments concisely yet persuasively and authentically.
An essay should also show a clear path of thought. Even essays written using the stream of consciousness technique have a certain goal at the end of the essay. One should not include material or pints in the essay which have little to do with the argument or with the topic. The argument should be structured in such a way (and the essay as well) that there should be a certain convergent tendency inherent within the points.
While writing an essay, one can go through previous exemplars dealing with similar topics. While composing an essay, however, one should take caution not to indulge too much in hackneyed ideas or clichés. These are some of the tips that one can keep in mind while writing an essay.