well, I did it in year 11, and my exam was about 9 days after the first two, so I had a good long time to memorize everything I could.
Learning concepts is very important, because as far as programming goes, the most they can ask you is to find a problem in some pseudocode that they provide, which if you have a good basis in programming shouldn't be a problem (it took me about 15 seconds to find the problem with the algorithm on the 2007 exam)
This website helped a lot:
http://www.mckinnonsc.vic.edu.au/vceit/main.htmJust basicaly if you ever get a topic set in class (like say Information Systems) go home and do a bit of research into them and the different ways they can be used. Just try to have a good basis in the theory of ITSD but also a good basis in programming (which I suppose is why they call it IT
SD 
)
And as for the 48, probably luck & huge crammage