Gentle stretching of the entire body. Start at the head and neck and work down the shoulders, chest, lower back legs etc. By starting at the top and working down the body, one is less likely to forget a body part. The stretches are soft and not forced. The key to juggling is to be relaxed.
Breathing is incorporated into the stretches appropriately.
I pick my favorite exercises. Anything will do if it gets the heart pumping. This is the big one that help me so much. Much of the tension that I felt in my wrist was due to tension originating from other parts of my body. By doing relaxing movements before juggling, pockets of tension that are carried by the body can be released.