The best prep of course is just paying attention and reading during the rotation. Having done that, the Boards and Wards IM section is good. For questions, MKSAP is the best as the difficulty level is appropriate; Harrison's board review is good though difficult, but you may benefit from going through an important section or two (e.g. cardiology).
Questions, questions, questions. Make sure you use MKSAP 2 for students (the blue one), as I was told the older green MKSAP for students contained discarded resident MKSAP questions that were too difficult for the student level. I went through MKSAP 2, did Pre-Test right before the exam (last 1-2 days), and read through most of the IM Clerkship Guide during the rotation.