If you have trouble with hands tensing up, I'd recommend slowing the pace a little bit, and trying to play with as smoothly as possible.
Mess around with your key layout, and try to find a comfortable hand position.
Also, be careful with your scrolling speed, I see a lot a players who can't manage their own scrolling speed, thinking it helps reading patterns, but resulting in unnecessary finger tension and hitting notes inconsistently early or late (often late).
Take breaks often, I rarely play more than 1 hour per session.
When you're bored you can also try to do fast one-handed trills on a table (alternating between two fingers). Try to do this as fast as possible without tensing your hands.
Also remember that tensing your hands during gameplay should only be used in death parts (Identity Part II notewall in o2jam).
And last advice, usually, sometimes not playing for a few days / a week, helps building greater skill when you get back at the game. It works for me and most people I know.