It doesn't hurt to prepare for a rotation, but its not necessary. Its better to be someone who takes the initiative to learn. Its easy to get complacent and get away with doing nothing. Keep a open mind, open yourself to new projects, and try to push yourself and your workload. In a good rotation, you will learn alot just by participating, and having a good amount of homework.
As a preceptor there are 3 general things I hate:
1. Guessing at questions. Don't know it, look it up. Preceptors give students questions mainly because we know that the student doesn't know, and its a polite way of giving you homework, so it doesn't hurt to say "I don't know, but I will have the answer tomorrow".
2. If assigned homework or given a question to answer. You should do it as soon as you can, and tell us what you found out before we come hunting for the answer. Students think that preceptors forget. No we don't (well sometimes we do). I had a student who said that she would do all these wonderful projects and reports, and I would assign her homework questions after a presentation. I don't think she ever did any of them, unless I brought it up again, and asked for it at the end of rotation.
3. Until you pass the Naplex, you are not god's gift to pharmacy. So leave the ego at the door please.
Treat all rotations as a extended interview. Even if the facility is not hiring, it never hurts to get a good recommendation for your resume or CV. I will always pass a student no matter how horrible or good they are. Giving references is a whole different ballgame.