The biggest issue is not answering the question, or tangentially brushing on the question.
No matter how hard the question is, just answer it. You might not have a fully developed position on the future of the US Postal Service (2012 prompt #1), but pick a side and argue it throughout your essay. Straddling the fence, talking about the USPS in general, or talking about something tangential to the issue such as mailboxes -> poor essay score (scores 0 - 3). Don't go down this route. My recommendation is just to take the first route delineated below, especially if your M/C section is decent. The pressure is high on test day, and you likely won't have much time to craft a multifaceted thesis. And remember, you have three essays to write in total; don't invest all your time in one essay. 3 average essays is better than 1 exceptional essay and two 1-paragraph "essays." Nevertheless, if you know everything about USPS, and can take a nuanced stance on the issue, go for it
Taking a side and arguing that side -> at least
an average essay (scores 4-6).
Taking a side and arguing that side with tact -> exceptional essay (7-9).
Crafting a multifaceted thesis without
coming across as straddling the fence -> exceptional essay (7-9).
Another mistake is to respond to any quotes or stimulus material that might be given to you as part of the prompts. Don't respond to the quotes. Respond only
to the question posted by the prompt. The source material is only provided for you to chew over, not to respond to. See the 2012 FRQs, prompt #3 for a salient example.
2012 Lang FRQs: http://apcentral.collegeboard.com/ap...g_language.pdf