Johnnyb, all great advice above. Let me add my 2 cents. The best use of your time depends on your weakness
, nobody has the same strength and weakness. Every entering student is somewhere in between; 1) super prepared in Maths or Physics and the like, and 2) maybe coming from Finance, knowing the industry having taken many finance courses but weaker in mathematical and statistical tools. All the worries I hear is what / more Maths to do , what textbooks to read before the Fall starts. It's OK to some extent if this is your weakness but as the above posters says, don't come in September already burnt out on studying. Work to your weaknesses. If you are a Maths/Physics type and especially if you did not study and did not live in the US, your major weakness is that you do not visualize the industry and the economic environment. It is very important for many quant jobs to understand the economic environment of markets, trading, risk management. You can't build this culture in a couple weeks before the first interview. If this is your case: Start listening to economics and finance programs regularly, read the finance / macroeconomic pages in WSJ, Financial Times or any very good newspaper, listen to NPR's "Market Place" for example. Snoop around the web sites of the major companies in investment banking, asset management, trading. They are full of fantastic links with white papers, "our strategy, careers, positions posted, about us, the culture, what they do, etc... This will give you a picture of the industry. If you are already in the finance industry, this may not be so much of an issue for you.
Also prepare your CV according to the US standards. If your program has career coaches, send it to them early in the summer so you can revise it on their recommendations. As Andy says when the courses start, it will be much harder to do that in addition to the courses.
For example at BU, we email our entering students our resume template and recommendations at the beginning of the summer and engage them to work on it and send it to us for review. For the harder stuff, we have two online summer review courses,
Python and Math/Stats, so everybody has reviewed the minimum needed knowledge.
But time and time again, I see entering students neglecting their "soft" preparation and panicking about their "hard" preparation.