Hi Luke
Have you used this website?
http://www.psychometric-success.com/...ty-tests-cubes.
Personally, I think the best preparation is practicing all types of spatial puzzles and not just cubes. If you search the net you'll find all kinds of free games and puzzles (I can post some links once I dig them up out of my inbox, if you like). I've found that practicing all kinds of mental manipulation really conditioned my mind...eventually you might find the cubes easier than the rest you're practicing on!
Best of luck!