One thing you can do is to move your legs a little bit. If you haven't been ordered to be completely still, try shifting positions so that you remain in a kneeling position, but can move the muscles to keep them from falling asleep. You can also try just gently tensing and relaxing the muscles in your legs as you kneel.

For some people, kneeling isn't a task they can do, especially if you have circulation or muscle problems. Try to find a position that works for you that pleases your Master, and also recognize that it will get easier with time and practice.