This is how your hand should be when picking up the cards
First off talk about how how balloons get static when you rub them against something like your shirt, claim you can do that with your finger and make it stick to anything, take your index finger and rub it against your shirt or another piece of cloth.
Now take something like a pack of cigarettes, or a pack of cards, then holding the cards or cigarettes with your thumb and index finger, and touch the pack a few times like its not working.
Then on the third try, make sure you press the pack against the knuckle of your ring finger and stick out your pinkie, it looks like the pack is floating but its just resting on your pinkie. The best way to do it is your hand tilted down but not far enough that the pack falls off.