Study like you would any other major exam, practice a lot, do lots of questions. Hardest trials are probably BOS trials, aka. the trails made by this forum. Regarding schools, it's hard to give an exact answer, it depends on the leniency of marking by teachers. Sydney Grammar are pretty tough in their trials from what I've heard though. It would eventually be best to just do past papers from your school since they are probably going to be similar to what you will get.