Before you begin typing, take a few minutes to plan what you are going to write. Pay careful attention to what the argument's conclusion is, and the evidence used to support that conclusion. Every proposal on the GRE and GMAT Argument Task is intentionally flawed, so look for faulty connections between the facts and the author's conclusion.
For every instance of flawed reasoning you find, you should explain how it weakens the argument. Each flaw should be described in its own paragraph. Other sources of flaws can be overly vague language, or misinterpreted statistics.
A solid essay will contain a short intro, 2-3 body paragraphs, and a conclusion where you briefly describe ways to make the argument stronger. The form can vary somewhat, but the ingredients for high scoring essays are the same. The GRE and GMAT essay graders look for organization, clarity, and development. High scoring essays have good transitions and avoid overly general statements.
Make sure you pace yourself so you don't run out of time. Also, give yourself a few minutes to proof read. The complete list of possible argument topics is available on the test makers' web sites. The GRE topics can be found at gre.org, and the GMAT topics at mba.com. Good luck studying!