There is some good advice here. I particularly agree with people saying that studies can only take you so far.
In terms of improving stamina, at risk of sounding obvious, building up playing time helps, but it can enforce bad habits unless your technical basis is sound. I particularly like what Seika said about thinking the right sound when playing, even before playing really, and playing slow is essential!!! For building up stamina and even-ness, I also found that playing passages (especially repetitive runs) in dotted rhythms very helpful.