Make sure you warm your hands and wrists up before coloring by doing some simple exercises. Rotate your wrist ten times clockwise, then repeat anti_clockwise. Clench and stretch your fingers as you unclench your fist _ do this ten times to get the blood flowing to your hands and finger_tips. This will help avoid your hand cramping as you color. Take regular breaks and repeat this exercise, I try not to color for more than 20 minutes without shaking the stiffness out of my hands!
Remember, these techniques take time and practice to master _ don't expect to be producing masterpieces over night, after all it took Michelangelo 4 years to finish the Sistine Chapel so give yourself at least a good couple of years of practice to produce your first coloring masterpiece!