How can I relieve the "charley horse" kind of cramp?

Don't remain in one position too long. It is easy to get hooked on web surfing and remain in your chair for hours. That makes for aches. Make yourself change positions, or, better yet, get up and move a bit every 20 minutes or so. Some people use one of those large rubber inflatable exercise balls as a desk chair - they can bounce and get light aerobic exercise while seated, at the same time avoiding stiffness and encouraging better posture.