Not having been there myself, I can't give advice from a "been there, done that" perspective. However, I will tell you what advice I have been given, and what I am doing. One (and only 1) program director talked to me about reading a little before the start, so I am reading a few pages 1x/week, starting this week up until probably May, in a text book that is a little more than you would need for the clerkship, but not quite a big thick text. I am just doing this to refresh my memory, because almost my entire 4th year has been a chill out time, and I don't want to forget everything (already have forgotten stuff since step 2) and be a big idiot on day 1, so that's my little paranoid fear. I may also (if I can really get motivated) look at some surgical instrument flash cards (ob) so that I can feel a little more prepared should i start with an OR rotation first.
Lastly, I'm reviewing my Spanish, and this is probably the most worthwhile of all that other stuff, cause I want to be able to communicate fluently with my patients.
I wouldn't stress about it, but if you've been chillin' like me (since October) then maybe it wouldn't hurt to make sure you remember how to give IV fluids or something. If you've been working hard all year, then I say relax! But again, I don't have experience. Hopefully you'll get some advice from people who actually know what they're talking about.