I do essentially what some posters have recommended, but definitely don't end up within the "page limits" set by some.
I try to start outlining my classes about two before exams (you could start earlier and do it more as you go). This ends up being a VERY long outline (60-80 pages) because it's essentially a reorganization/integration of class and reading notes.
Then, about 2-3 wks before finals, I start cutting down, condensing, shortening, etc. For open book exams, I usually end up with something that's 40-50 pages.
How long your outline ends up is really up to you and what you find useful. For open book exams, I print my outline out, tab it, usually end up writing in the margin small notes, etc. and am familiar with every single page.
Some people say 40-50 pages is way too long (in fact, I believe I've been told on this site that 50 pages is "way too f-ing long and it's ridiculous). It works for me though. I do actually refer to my outline A LOT during an exam. It's worked out extremely well for me. Some people may not have time to refer to outlines (I'd imagine after my post, someone will chime in with that comment). I do have the time, and I get more As than anything else, so it works for me.
So don't hold yourself to any certain page limit. As you get closer to exams next fall and are working on your outline, consider the parameters set by your prof (one prof only let us bring in a 5 pages total), and what you think will be most helpful to YOU. And go with whatever you're most comfortable with, whether its distilling it down to a 5 page outline or going in with a binder holding a tabbed, highlighted and noted 50-page outline.