No way to Prepapre for Mensa!
There is no way to study for a Mensa or IQ test. They have nothing to do with general knowledge or things you learn in school. It is all about your thinking and reasoning skills. Either you have them or you don't. An IQ is inborn and there's nothing you can do to raise it.
The best things you can realistically do are:
eat right, sleep right, avoid big meals or medications that will make you drowsy and relax. Feel good and be alert. In fact, try to take a walk-in test that you can choose at the last minute. That way, if you have a headache, illness, etc., you aren't tied to a specific date.
You can't study for these kinds of tests, so the most you can do is make sure your head is clear and your brain is in good shape.