I'm sure amphetamines or methylphenidate work even better, but I use an ECA stack (25mg ephedrine/150mg caffeine/100mg aspirin) for study aid. Does a good work on improving concentration and there's not really a comedown or side effects. It has helped some times when I had no people to study with (best option ever, if you find someone you are good at studying with, it's the best option out there, In my opinion). If I'm alone I'll just read for a few minutes, stand up, fiddle with something, sit back down, read, repeat...