When composing your sentences, keep most of them as simple as possible. Sentences that contain several ideas, are too long, or are complicated are the most difficult for your readers to understand. Try keeping most of your sentences short and to the point. Each sentence should have a noun, adjective, and a verb and should convey one main idea. If you follow this rule, then you'll express ideas fluently and precisely, using effective vocabulary and sentence variety, which is one of the criteria that is used to determine the score that is awarded to an essay. So, you'll want to include compound, complex sentences in your GRE essay, but most of your sentences should be simple and straight-forward, because then you'll be assured that your intended message will be received as intended.
Consider the following examples:
I was very excited to be leaving home on my way to college. I was not just happy to be going to a new place but I was also going to be learning new ideas and meeting a large variety of new people which would hopefully lead to many fun and interesting new experiences.
The second sentence is a listing of several smaller ideas that have been combined into a sentence that is too long and difficult to follow. Watch what happens when the ideas are split into several shorter sentences:
I was very excited to be leaving home on my way to college. I was not just happy to be going to a new place. I would be learning new ideas and I would be meeting a large variety of new people. I hoped that all this would lead to many fun and interesting new experiences.
The same ideas are present in the second example but they are each given their own simple sentence. This construction is much easier for the audience to follow because it does not involve remembering a list of items from one extremely long sentence.
Keeping your sentences and your essay construction simple is one of the easiest ways to ensure readability. Overly complex word choices and/or sentence construction will only alienate your audience. Remember, you want your audience to understand both your purpose for writing the essay and your specific message. Do not risk confusing your message with complicated word choice and/or sentence construction.