Aimee Yermish's article on Hoagies is great to read before an IQ test: http://www.hoagiesgifted.com/test_prep.htm

Our DS was younger when he took his first IQ test, and we just told him that this was a fun test that would help us see how his mind worked, which would then help us decide where he should go to school. We told him to make sure to cooperate and answer all the questions, even if they seemed really, really easy and even if they seemed silly, since the more interesting questions would only come along if he answered the easy ones. (This was mostly a caution because at the time he was refusing to do things he thought were too easy.)

I think most GT kids think IQ tests are fun. Hopefully yours will too! Try to get her a good night's sleep and a good breakfast ahead of time. Send easy to eat snacks along.