Also, do what you can to reduce stress in your life. Stress will carry over into your practice session and cause you muscles to tense up. If you can't change the things that cause stress in your life, change the way you react to those things. Oxygen is wonderful stuff, your muscles and tendons will agree. Fill you lungs with oxygen when you shake those arms between drops.
Fit muscles warm up faster and respond better.
They are also less prone to injury. I highly recommend aerobic exercise. Exercise physiologists and sports medicine professionals are now discovering that moderate exercise such as walking at a brisk pace can be just as beneficial as intense aerobic exercise. Unicycling is an excellent complement to juggling and provides excellent exercise as well. After juggling, walk around, shake those muscles, stretch them gently, let your heart rate come back down to earth.