I struggle with the same thing....but playing Bach has helped me IMMENSELY. His keyboard technique is so graceful, it's like dancing.....if you approach polyphonic Bach with too much tension, the technique will never come.
It's just something you have to constantly be aware of as you are playing. I have to remind myself all the time to relax. It is most difficult to keep the tension out when I am playing passages that I haven't quite mastered yet, at a tempo that I can't quite yet handle.
Also, keep in mind that you can use gravity to your advantage in piano playing. Rather than tensing your arm up to get a good sound out of the keyboard, relax it, and let it drop. In the fancier Bach stuff, with detached articulation, I like to get some good distance from the keyboard before falling back down again. It distressed me, watching Glenn Gould play Bach, because his hands are always so close to the keyboard, and I thought maybe my approach was bad. But then I watched András Schiff play Bach, and his hands often come up so far that you can see them come up when the camera is on the other side of the piano! Also, there is a good video of Martha Argerich playing Bach
, where you can see the technique....it's often just a matter of taking your fingers away from the keys for a split-second so that you can rearrange your hand completely without any stretching or awkward bending. I think GG is just a freak - I don't know how he plays Bach that way.
Piano students who are raised in the primarily-legato school of 19th century piano music often have the same problem.