Read the rulebooks completely, over and over again. You can never read them too much. Keep your tests for the associations that you are calling for, and study them over and over, especially the questions that you missed. Get casebooks for both ASA and USSSA, and study the similar plays and the ones that are different in how they are handled. Did I mention that you can never read the rulebooks too much?
I have heard of people using the cards as Elaine suggested, and they seem to have success with that. Either way, the first couple of years of umpiring consist of a lot of reading and studying. Once you are in the habit you will find that it will help you out when you are a 5 year veteran or a 10 year veteran....etc.
Good luck!
Scott