Juggling balls that are over five times heavier than your regular set is a good way to train muscle memory and increase fitness in your arms. The most noticeable effect of juggling heavy balls is when trying to build endurance with five balls.

You can buy heavy balls especially for juggling, called Pow3r balls or Exerballs. These are quite expensive, so why not make your own?

Ingredients:
5 tennis balls (old or new)
£4.00 worth of 1p and 2p coins (or other low denomination coins in the currency of your choice)
Electrical tape (a small roll)

Method:
1. With a craft knife, cut a slit in each ball, not much longer than the diameter of your largest coin.
2. Insert 80 pence worth of copper coins into each ball until they are full.
3. Wrap tape around the ball, covering the slit.

And there you go. You now own a set of the most useful juggling training aids ever invented. To make the balls heavier you can fill them with lead shot, though it can cost quite a bit when making a set of 5 balls, and the heavier the balls are, the more likely they are to split after many drops. A bit of strong glue on the cut goes a long way to making them a lot stronger.