I know where you're coming from. I don't think there's any one real way to prepare for a postbac before you know what kind of classes you'll be facing (the teacher's methods, the textbook chapters that will be covered, etc.) but I've found this helpful: if you already know what kind of book will be used in the course, buy it early and study it. That way you won't be suffering with a heavy workload right away because you'll already be familiar with much of the class material.
Also, if you know you're weak in a particular area, get a recently published book on the subject and read up on it. As I've mentioned elsewhere, I was astonishingly bad at algebra. A book wasn't enough to save me so I needed a tutor, but for most folks a book should be enough. If you still need a tutor when you get to class, at least you won't be as clueless as you would be going in without any preparation.
Hope that helps.