Just recently, I've decided to keep a "journal" documenting the little things about Latin (and Greek) that always trip me up. These things might include tricky bits of grammar or words that I tend to mix up. One of my entries, for example, explains the differences between ab, de, and ex.
I bought a Moleskine (only the best!) for this endeavor, because it's easy to carry around with me and fits nicely into my back pocket.
Anyway, I'm really pumped about this and I was wondering if any of you do anything similar or have any suggestions.