In general, try to sound sincere and convincing. Bring your student ID and at least two pencils and two pens with you. The pencils are for bubbling in an information questionnaire before the test. Ink is officially required for the test. Don't think that failing the GWT automatically means you are a poor writer.
Comparisons generally ask for similarities more than differences.
Definitions are usually short. Give concrete examples. If your grammar is fine and assuming that you organized your essay well and included specific detail, you should revise for unity. This means you must make sure you have stayed on the topic. Suppose the assigned topic was to discuss a habit you have and how it either positively or negatively affects your life.