Even though they are blocked in a neutral position while playing, the muscles are still working to hold it there, so gently flex and extend your wrists as well moving them in slow circles a few times. A brisk rub up and down as well as around your wrists, ending with a vigorous shake will warm up the wrists. Your elbows are held in flexion and abduction or raised. Using your other hand, squeeze the muscles of the forearm and upper arm a few times. Shoulders are curved forward as well, the great pectoralis muscles doing the work of keeping those elbows up and bow arm moving.
Shoulder rolls forward and backward are helpful if they are done gently and slowly.
Don't forget to open and close your mouth a few times too. Lots of people hold stress in their jaws, clenching their teeth when concentrating. By this point you should be feeling pretty loose and comfortable, ready to play. At this moment, stop and remember how it is you feel. You want your brain to remember this pattern, so it can alert you when tension or muscle imbalance begins during your playing, signaling you to correct it.