Kneel on an exercise ball (see tips on previous slide). Once you have your balance, have a partner gently toss you a ball to catch. Throw it back. To really work your core and balance, have your partner vary where they throw it so you have to twist or reach slightly to catch it.
Training tip: You can do this with one (or both) partners kneeling on the top of a Bosu while you're learning to maintain your balance.