I always find myself needing more juggling balls- a friend has "borrowed" mine and kicked it flat playing hackeysack, I lose one in the airport, I learn how to juggle more balls...
Though you can make do and juggle nearly anything, I really like to juggle with nice, crocheted balls. And when you make your own, you can make them exactly the fullness, dimensions, and weight you're looking for.
This tutorial also works for hackeysacks.
Step 1: Materials
You need some string, a crochet hook, and something to fill the hackeysack with (more on that in the "filling" step).