This coming from a student who is a long way from a dan grade. But my observation thus far having observed people in my own dojo preparing for dan gradings. (our most recent was a nidan test) is that it would be a good idea to know who you will be testing in front of and what they are looking for in a test. Your sensei should know this and help you with preparations. Other than that, just get on the mat and do good aikido.