I really appreciate the following advice (Re: The GRE discussion) from our codenamer of the week: Incredibly Undulating Daughter:
I used to work in a testing center, and this is pretty common--especially if you study books and practice tests designed by ETS (the company that gives the GRE).
It's best to purchase guides from Kaplan, Princeton Review, or other reputable third party sources. Remember, these are corporate entities. ETS wants you to have to take the GRE at least twice. On the other hand, it's in Kaplan's best interest for you to do well on the test and tell all your friends. Never assume a school or a testing service has your best interests in mind just because they're involved in education. They don't. Everything is run like a business these days.
Beyond that, your scores will almost always be lower than your practice tests due to nervousness. Then there's the unfamiliar location, which is usually too hot or too cold. This has also been shown to affect scores.
Bottom line, the best way to prepare is to start early, study non-official, third party materials, and familiarize yourself with the test location in advance.