At my last school I had an instructor who insisted that we get up and stretch every 15 minutes. Keep your back straight, both feet flat on the floor. Make sure your monitor is at eye level so you don't have to hunch or bend your head. Mine isn't adjustable so its propped up on a stack of books. Keep your wrists straight while you're typing and mousing.
I don't know if anybody else does this, it might be just me, but when I'm using a tablet I tend to tighten my grip on the pen until my knuckles go white. Every now and then I need to give myself a shake and make myself relax.
I know a really good back stretch which I shall try to describe. You stand up straight with your shoulders back. Extend your arms back and flex your wrists at a 90 degree angle, palms down. Press back behind you with the heel of your hand (you're not actually pressing on anything, I just mean that to be illustrative of the gesture). Then twist your arms so your elbows face in, then out. Then roll your head on your neck in each direction.
This stretch flexes all the muscles in your hands, arms, back and neck and it feels really good. Fair warning, it occasionally makes me go light headed. I was worried my description would be confusing so I I did a stick figure of it which I have attached here.