Do you have any good exercises to relax before playing the piano.
I'm interested specifically in relaxing the arms, shoulders and back.
@Heather: great advice about relaxing! (I wanted to post this as a comment, but it is too long, by far). I found doing the "windmill"-stretch really good (and neccessary)
This helps stretching your back diagonally, and also do the "reaching up, plucking apples". Basically the back is nowadays everbody's Achilles' heel, so also work on your core strength (abs and back) - and this goes for most people (and instrumentalists).
See this https://www.classicfm.com/discover-music/humour/yehudi-menuhin-yoga/ for a famous violinist, who said that "his yoga teacher was his best vioin instructor". He also wrote a book "The king, the cat and the fiddle" which includes stretches and exercises for violinists on the last pages.
Edit / added: I personally find the reply by Heather S. more "on topic" than this one - even if mine is marked as "the answer" (as of Oct.2018).
I have found two ways of relaxing helpful.
One is to first tighten the muscles you want to relax, then relax the muscles. It helps to really be able to identify the feeling of tightness, so you can undo it as soon as you notice it.
Another is almost meditative in quality. Really let yourself feel gravity pulling on your muscles, almost like you are creating a "dead weight" feeling. A person's muscles are most relaxed when asleep. I don't know if you have ever picked up a sleeping child or cat, but if you have you know how they are kind of "limp" and a bit extra heavy. This is because their muscles are doing no work to help counteract gravity. Try to get this feeling in your arms/shoulders/back. If you don't fully feel the "dead weight" it is because you are still using a muscle to hold yourself in place. You won't play like this, but it helps to know the "extreme" feeling of relaxed. Practicing getting to this extreme can help train your mind to "go to relaxed" as soon as you notice tension.
You may want to try this second method first while sitting in a chair with a back. I have gotten relaxed enough to almost fall off a piano bench before. Like I said, it is a bit like meditation, and one can really zone out.
When I am noticing that I am starting to develop a bad habit of tension in my arms again, I practice what I call "zen scales". When I play these, I play scales as slowly and softly as possible, moving each finger (using proper fingerings) with as much of that "dead weight" feeling as I can.
I know stretching helps, too, but I don't have specific stretches to recommend. I do stretch, but it's mostly what feels natural to me to do, as if I was stretching after getting out of bed in the morning. I find I need to stretch again after doing my relaxing, almost to reawaken and get down to business.
Hope this helps!