This topic has come up several times and I'm reposting some things I've written before. I don't think that it is ideal or necessary to spend the whole summer cramming math, and I'll re-post my old message, which explains why:
A friendly word of advice -- whatever you do, don't arrive at school burned out. If it's been a while since you were in school, or if you haven't taken linear algebra and real analysis, you may find it worthwhile to work through some math excercises. (Simon and Blume is a great review, and many of you will use it in math camp as well.) Similarly, if you've taken a lot of math but very little econ, it may be useful to take a peak at an intermediate undergrad micro textbook. (I'd recommend Varian's undergraduate text, or, if you're a little more comfortable with the material, his excellent graduate text, simply called Microeconomics.)
However, don't spend all summer studying! Don't read Mas Collel. Don't bother with macro at all. Let your professors guide your approach to this material when you start graduate school, and give yourself a chance to come at the material rested and enthusiastic. It's very easy to get bogged down in the details if you try to learn MWG in advance. For that book especially, it's much more productive to treat it as a reference than a primary source. Try to get the intuition from your professors, classmates, Varian, or Kreps (another good grad level text). But don't beat yourself over the head with MWG over the summer. You won't be behind if you haven't already read the book, and you won't be much ahead if you do.
What should you do to jump start your grad school experience? Start looking for interesting questions. Train yourself to look for research topics in every day life. Save newspaper or magazine articles that make you say "huh" or "that can't be true" -- they may end up generating research ideas in the future. Keep an eye out for great natural experiments, policy changes, instruments. When you have ideas, write them down. However silly they are, such a list is a place to start when, suddenly, you're told to "do research." Remember that economics is a fantastic tool for analyzing all sorts of behaviors. Pay attention to, and be interested in, the world around you.
If you want to do something a bit more rigerous, then I'd suggest getting someone (an old professor or current grad students who are a bit further along in their programs) to suggest a half-dozen seminal papers in the fields you think you are interested in. Read these papers. You'll read them again when you take field classes, and another time when you start your research, but reading them now will give you a better sense of where you are going, why all of the first year theory matters, and what research questions interest you. It will give you a little bit more background when you go to seminars and talk to other students about your research. And re-reading the papers halfway through the year is a great way to see how far you've come in building your foundation as an economist. Engaging with the literature in your intended fields is, IMO, much more useful than slaving over MWG. There will be plenty of time for that
Something else I posted previously on the subject. This is a bit redundant, but I sill think it's true:
Well, I can tell you what is NOT the best way to prepare for first year: trying to cover all the first year material by yourself over the summer. :no:
Having a PhD already is not a prerequisite for enrolling in first year classes. They really do plan to teach you this stuff starting in September. And it's much, much more productive to learn from your professors and classmates than by trying to read MWG. At best, you will get frustrated, and at worst, you will get confused.
If it really has been a while since you've done any math, then reviewing basic calculus (multivar, total derivatives, etc.) is useful, and your professors won't spend time on that during math camp. Similarly, linear algebra is a basic tool that shouldn't require much though, so working through old notes or problems could be helpful. But for math beyond that, especially math you haven't ever learned before, I'd suggest just waiting. Self-study of real analysis or dynamic programing is frustrating and low-yield. If you have strong basics, you will learn these things when school starts. And it's better to come to them fresh than confused and frustrated.
What you should do, in my mind, is start thinking like an economist. Subscribe to the NBER digests as soon as you have a .edu e-mail address, and read a few papers that sound interesting to you. Start keeping a notebook of research ideas -- things that you see in the paper that make you say "hmm, really?" or other ideas you stumble across in daily life. If you don't already, start reading the Economist. And find a few academic articles (ideally, the seminal articles in your field, but if you can't get anyone to suggest those, then settle for papers presented in the seminar series for your field last year). Read those just to get an idea of what the end goal of the PhD really is, and to give yourself an idea of how the first year course work will fit in to the type of research you want to do.
Other than that, enjoy the summer!
(You'll also find useful advice from lots of other TMers on the old threads I've linked to. A search might turn up even more. This subject comes up pretty often.)