My Kaplan practice tests predicted my MCAT scores exactly with one pleasant surprise. In the Kaplan tests I was running 9s on Verbal, but when it came to test day I scored my first 10 on verbal and 11 and 10 on Phys and Bio sections. I don't know how they do it, but there magic worked for me.
Each Kaplan test has a different degree of difficulty. Some have notoriously hard physics sections, others tough verbals. But practicing those full-length tests for three or four weekends before the real one is, I think, a great way to prepare your brain to handle the stress of test day. Also, try to take the exam in a familiar environment (your current school, e.g.) and try to visit the room before the test just to get a feel for your surroundings. And don't forget food and water for the day. Don't depend on the school's cafeteria to be open.
So, bottom line, practice, practice, and practice, and then show them what you can do on test day.
Good luck to all.