Here is a tip on how to clean ball bearings. First find a small plastic container where you can place the ball bearings.
I am use a plastic film container used to store camera film. The ball bearings are then sprayed with WD-40 oil and I fill the container with WD-40 just above the bearings (about half full)
Next cover the container then vigorously shake the container. This will loosen and remove the dirt trapped inside the bearings. Remove the bearings from the container, dump the dirty oil, and repeat the process (spraying the bearings with WD-40) until the oil is clear even after shaking the container.
Tip: WD-40 is really more of a cleaner with an oiling and anti rust property. If you really want smooth bearings, use light oil (e.g. Trinity oil, Triflow) to lubricate the bearings after they are clean. Usually a drop is all that is needed.
* There are also other ways to clean the bearings and store bought cleaning tools that work great.