A few things that worked for me when I was trying to get the feeling of being relaxed in my head:
The simplest way I've learned to release tension in the upper body is to do some simple boxing combinations; jabs, hooks, backfists; and hold a bit of eggshell in my hand while I'm doing it. If you're too tense, then when you punch you end up crushing the eggshell - but once you know what relaxing feels like and that clicks in your head, then you've got something you can train in. (Boxing is mostly a legs sport and if you're tense in the upper body it doesn't work very well.)
I also, from time to time, imagine I'm holding a steering wheel. When you do a lot of driving you end up relaxing your arms and dropping your weight onto the wheel to smooth out the imperfections in the road that you can't really respond to at 70mph. It's similar to when you're holding a sword in front of you and someone tries to knock the point away - if you're relaxed properly, the point just comes right back to true and suddenly they've got a sword in their face.
You might also try leaning against a wall with your arm flat against it and pushing off against it with that arm for a while. When you step away from the wall and relax your arm it will naturally tend to rise a little if you're relaxed properly.
Don't know how applicable any of those might be to you. I found that when I was trying to relax I didn't really FEEL the tension, so someone would tell me to relax and I'd think - well, I'm not tense. But, once I'd found that feeling it was something that I could look at in my techniques and remove relatively easily just by doing them slowly remembering to relax whenever I found myself tensing up until it eventually became just the way I did it.
The biggest things I could say would be to try and find that feeling and to take everything slow. It's really difficult to concentrate on building a particular aspect of skill when you're trying to concentrate on not getting your block knocked off.