I have been told that the 'correct' posture for juggling
is :
feet shoulders width apart, side by side
knees bent slightly
back straight
arms at side of body, only bending at the elbows
Personally, I don't believe there is a correct posture for
juggling. If you can do it standing on your head, then good
for you. You might find the above useful when you're first
learning though.