Tallguy, theres a book called Get A Head For The Sky that is highly recommended. Its meant for wannabe pilots but the principles are the same for ATC. Whats more the interview advice is brilliant. Within that there are loads of addition, subtraction, multiplication and division examples, as well as percentages, conversions etc.
Other than that, doing mutliplication tables is never a bad thing.
Re: cubes, see my post from a couple of days ago!