There's also a great one where you ask the parents to send in an encouraging note to their student on the first day of testing. I don't have it to attach from my netbook but as soon as I get home to my documents folder tomorrow I can attach it, too.
I posted a letter here that I sent to parents asking them to write a note of encouragement to their children before a test. It is available in both English and Spanish, since I work in a Spanish-speaking community.
It's written in MSWord so you can change the information that is specific to your school and to your students.