Once a purpose is established it is important to think a little bit about your audience. Who will be reading your GRE essay? While this may at first seem simple (a teacher, an evaluator, a trained GRE employee), resist the urge to simplify your audience in this way. Keep in mind that who your audience is will have a dramatic impact on the way your writing is understood. If your audience is removed from you (an evaluator, for example) it takes even more care to be sure you don't alienate that person in the course of your essay.
An essay that does not take audience into account might look like the following:
When my friends and I were hanging out that day we were listening to some tunes. I was working on my carburetor because it was running way too lean. My friend John had been knocking back some brew because it was wicked hot and he was running his valves in the sun.
This example would be difficult for most people without technical knowledge of the author's activities to understand. While most people know what a carburetor is it would be unwise to assume that the audience could identify well enough with the events to understand what is going on in this passage. In addition to the technical language, this example makes use of a significant amount of slang dialect that could be easily misunderstood. The phrases "knocking back" and "wicked hot" are both regional slang that are not appropriate in an essay. Their use risks alienating audience members who do not commonly use those terms.
A better example of the above sentences would take into account the potential for misunderstanding from a reader without the same background knowledge as the author.
My friends and I had spent that day talking together and listening to music. I was working on my carburetor because my car had not been running well lately. My friend John had been drinking beers all day to cool off because he had been working on his car in the hot sun.
My friends and I had spent that day talking together and listening to music. I was working on my carburetor because my car had not been running well lately. My friend John had been drinking beers all day to cool off because he had been working on his car in the hot sun.
My friends and I had spent that day talking together and listening to music. I was working on my carburetor because my car had not been running well lately. My friend John had been drinking beers all day to cool off because he had been working on his car in the hot sun.
And, most importantly, you want to ask yourself, "What do I want the audience to understand about my writing?" Because on the GRE composition assessment tasks you are asked to write intelligently and with insight about an issue or an argument, you must write for an intelligent audience, and most likely you'll want to use few -- if any -- references to slang or regional terms.