For my aptitude test, the only thing I really did in preparation was a few free IQ tests online (surprisingly similar in some aspects). That, and I brushed up on long division (since I used a calculator all through high school, which obviously isn't allowed during the CFAT). As a result, I scored high enough for any job in the Forces I wanted; NCM or Officer.
That was all I did.
From what my Recruiter told me, you can't really "fail" the CFAT test. Lower scores give you trade choices like the Infantry while higher ones give you options like becoming a Pilot. If you don't score high enough for your desired trade however, that could be considered a fail. That being said, if you don't mind me asking, what trade did you apply for?