In addition to clearly having solid introductory and closing paragraphs where you summarize for both sides of the argument, you'll also want to provide ample evidence to support your stance. In fact, evidence is a very crucial component of the issue essay -- but not just any evidence. The issue essay topic questions that you will encounter on the GRE are broad questions which can be effectively addressed in many ways. However, you'll want to choose your evidence wisely.
First, plan to have at least two, if not three, pieces of evidence in support of your stance. By evidence we mean pieces of outside knowledge that help strengthen your argument. For example, if you are arguing on the importance of science fiction and how it helps shape future scientific research, you may want to include information on authors, on the idea of how the opinions of readers influence authors, which in turn influences how authors write, which in turn causes authors to produce books which future children will one day read, some of whom will become scientists and researchers. Here, then, you'll notice that in order to have an effective issue essay, you'll need to have evidence in favor of your claim, and so when you are selecting from among the two issue topics that you are presented with, you'll want to pick that topic that you are most familiar with and for which you can right away come up with examples to help strengthen your cause.
Furthermore, you'll want to include at least one body paragraph where you discuss any arguments which may be raised by a contrary view. This is an important part of your essay because it demonstrates to the essay graders that you are not only aware of your own hypothesis, but that you are cognizant of an opposing view.