as zheer was saying - to relax and relax and keep relaxing until you are playing with the least amount of effort to get the same sounds you get now.
My teacher tells me, and i know, that i need to have more stamina when playing powerful pieces at high speeds.For example, when playing the Revolutionary Etude, i start powerfully and with all LH notes crystal clear, and then gradually as i move on, my left wrist becomes more and more tense until i can't play like before, so notes become more blurry and unclear and i try to hide that by using more pedal, which is of course a very unmusical thing.This situation becomes more and more worse if i am playing in a recital or something, I won't be able to play all the programme because near the end, my muscles would be so tired.So, could anybody please tell me any guidelines or anything that i can do to build more endurance and stamina ?