Move any obstacles such as boxes or equipment. Avoid excess pressure on the backs of your legs and knees. A footrest, particularly for smaller users, may be helpful. Don't sit in the same position for long periods. Make sure you change your posture as often as practicable.
Some movement is desirable, but avoid repeat stretching movements.
Adjust your keyboard and screen to get a good keying and viewing position. A space in front of the keyboard is sometimes helpful for resting the hands and wrists while not keying. Don't bend your hands up at the wrist when keying. Try to keep a soft touch on the keys and don't overstretch your fingers. Good keyboard technique is important.