Yeah, I know there are warming exercises, but usually they don't ease much playing strain.
Actually, even doing exercise for the upper forearm won't help much neither, though it's for sure useful.

I do a lot of hand stretching AFTER playing (like you would BEFORE Aikido class, for different reasons), as you would body stretching after gym.
At least for me most pain comes from straining finger tendinis and consequently upper forearm.
Speaking with piano pros, the reason for this pain is just that I'm not enough trained, crystal clear.

 

Had to chime in here. If you are like me and are often pressed for time, just adding atleast 2 or 3 of these stretches after a session made the world of difference to my sitting in the past few months. Targeted to sitting muscles, based on academic research by Jeremy Ethier.

The PERFECT 10 Minute Daily Posture Routine (FIX YOUR SIT!) - YouTube

TLDR stretches (my personal favourites): Kneeling Hip Flexor stretch, Pigeon Stretch, Cobra pose and the "Stand and Reach"

Try the four I suggested at the end of a session and see how good you feel going to bed(no jokes going to bed was when I noticed the difference, I felt great). Just a minimal amount of corrective effort can make a world of difference.

As for other joint, back or arm specific pains: have a browse on Jeremy's youtube channel you will find corrective excersizes for your specific muscle/joint pain and I recommend experimenting with a few(I went to 3 different physios and a bio kineticist, all gave me similar advice to what he prescribes in his videos - i.e. "shoulder pain? do these three excersizes" etc).

Additionally I have enforced a mandatory 20-30mins cardio daily(at minimum a walk) to promote bloodflow as well as ab excersizes to improve posture. this combined with stretching has made the most difference.