I just started the first year of my PhD program, and kept tabs on this thread quite a bit while I was applying to graduate school/this past summer. I took a year off between undergrad and graduate school, and worked at a pretty mindless job for a year, so I was quite excited to start school in August. Lots of people say "just take it easy"... "easy" is not really a speed of mine so this was very challenging. I wish I would've had someone tell me something specific I could do, to help myself out in a few months. Here's some of those things!
- In Political Science (what my PhD will be in), R is the industry standard for data analysis. My BS is in Psych, and I know SPSS well. Learn the foundations/basics of what is common for your industry while you have free time.
- Learn some other programming languages while you're at it (Java) so that you can run your own experiments when you're ready.
- Get a handful of the most recent journal editions that are standards in your field. Many libraries have access to BrowZine - use it.
Good luck!