Resting for an IQ test means getting a good night's sleep, but more than that, it means resting your mind before you subject yourself to the rigors of testing. Avoid watching TV from the night before the test. TV may be a pleasant leisure activity, but it does passively sap the attention span as well as causing occasional eye strain.
Pre-test meditation exercises can be helpful; try closing your eyes and listening to the sounds of your surroundings, starting from nearest to farthest and trying to identify them before moving on. Just be sure to try your mediation in a quiet area or you may simply be generating stress.
Eating a healthy meal before an IQ test is important. Much has been made of children having difficulty learning on an empty stomach, and the same can be said of reciting what you have learned come test time. Eat a meal of complex carbs, complemented with some fruit for natural sugars and a glass of water to ensure that you stay hydrated.
Complex carbs are most easily found in natural grain-based products like oatmeal and whole grains. These take longer to digest, making for a slow release of sugar into the bloodstream and a more stable overall mental state. Eating a meal based entirely on sugar or fats can cause blood sugar levels to fluctuate throughout the day, and in the worst case you could become exhausted by a sugar crash.
The actual form of an IQ test can be difficult to predict and therefore difficult to prepare for. But there are a few things you can do during your test.
Divide your total test time by the number of questions you have to answer. If you get stuck much beyond the average time you should be spending per question, mark the question down, skip it and return to it at the end of the other questions, if possible. IQ testing is a game of averages, so sometimes knowing when to abandon a tough question can improve your overall mark.
While you are taking your test, be sure to check your breathing pattern; often people breathe shallowly or erratically when under stress. Taking slower, deeper, even breaths can help keep your mind on an even keel.