Preparing for the test depends on what you are trying to get out of it.
If you are just starting out i suggest reading a book that goes into detail. While you are reading the book follow along as much as possible using the computer.
If you need to know how to troubleshoot, best way to learn is by hands on experience.
If you already have knowledge of the subject matter the Passport books do a good job of explaining the material that they will be looking for on the test.
(If you are doing the A+ OS exam, may I suggest reading the first several chapters of the Network+ Passport book. It explains several concepts that are tested by the A+ exam better than the A+ Passport)
If you really just want to get certified, without really getting into the material or need a final quick study review before the test use braindumpcentral.com
Using A+ Passport and BrainDumpCentral to study, you will have seen at least 80% of the questions on your test.
Good Luck
-edluke