First, take a look at these ETS topic pools:
This is a list of ALL of the topics from the exam, so you WILL see one of these topics on your test. The prompts you will see on test day are drawn from these pools!
Write practice essays in response to some of these prompts. Even though the prompts you will see on test day are in those lists, don't try to memorize all the prompts. It's more important to practice the process of brainstorming, writing, and editing an essay in response to a brand-new prompt.
You can read the scored sample essays on the ETS website here:
Chris Lele has also written some sample essays to which you can compare your own essays:
We also have a bunch of resources on our blog, including an entire section dedicated to the GRE essays. Spend time reading the articles in the Writing section of the blog as well as actually writing essays on your own.
For even more example essays, I'd recommend checking out Vibrant Publishing guides to GRE Analytical Writing.
Finally, although Magoosh does not provide an essay-review service — we simply don't have the people-power to do so — you can learn about several ways to get your essays graded here: