How do I prepare for an exam that you're not supposed to be able to revise for?

The LNAT is unique in that you cannot revise for it from a book or using an online guide; the questions are designed to test your critical evaluation skills and your independent thinking. That said, you can prepare for it, and if you prepare well enough, you should have no problems when you're in the exam. There are many practice exams available online, and by timing yourself when you answer the questions, you can get used to the time limits in the exam. The important thing is that you read the questions carefully and evaluate the applicability of each answer to the passage you've been reading.