I'm not going to tell you any specific concepts to study, but would instead recommend that you do a lot of light reading across a wide range of topics. Don't worry about getting an in-depth understanding of each subject you read about - at this point, it's more important that you're able to recognize what kind of problem you're looking at, so that you can do some just-in-time studying when you're actually faced with it. In other words, it's ok if you don't know how to solve a combinatorics problem, as long as you know enough to look up "combinatorics" when you need to see how many ways you can arrange a set of objects or pick a subset.
Wikipedia is a pretty good resource for this sort of wide-ranging browsing, especially if you're just skimming to begin with. An even better one, especially if you find Wikipedia too academic or inaccessible, is the C2 wiki. (This is, interestingly enough, the original wiki invented by Ward Cunningham).
