One important area to add to Capn's very good node on learning how to juggle is the psychological aspect of juggling.

There is absolutely no physical reason why any able bodied human shouldn't be able to juggle three balls with their two hands after reading the above write-up. What is really important is the psychological issue of how to convince your simple disbelieving human brain that it's possible and very easy indeed - and once you've studied the movements neccessary and start to practise you must :

Stop Thinking.

Deceivingly simple, your hands must become detached from your brain as soon as the very slow embryonic rhythm starts, you don't need to think how to catch a ball, or how to duck, or how to have sex in order to do it well (I hope!). You just do it.

So if you've tried many times at juggling and have always failed, try again, start slow as possible and then transfix your mind on a incident of deep thought, a past memory that makes you very angry is usually the best way to both concentrate the mind elsewhere and to get some adrenaline pumping - which should imperceptively speed up the rhythm without you noticing. Before you know it the movement of your hands will suddenly become somebody else's problem.

Thinking Only Makes Life Complicated.