Stretch your hands and forearms by putting them straight out, and using one hand, gently pull the other back toward the elbow, first up, then down. Do it every time you save your work or take a break.
If you're really locked up, try a massage therapist. He or she can show you how to self-massage those muscles so that they 'unclench'. They should NOT feel like cords in your forearms, they should feel like warm worms or bread dough. Hard to describe, easy to feel with a little training.
Ron