Tecniques to ease muscle tension/contraction for better playing

[ruafo]

Hello.
I am currently practicing Prokofievīs piano piece Montasque and Capulet from the Romeo and Juliet, but i ned to work on some tecniques considering muscle tenison (not only on the Prokofiev piece, but on other pieces as well) when play i cant really relax my muscles in the wrists, fingers, biceps and shoulders at the same time. So my playing sometimes gets really stiff in the sound.
I will therefore ask for advise or techniques to resolve this problem