http://www.bocabearings.com/Issue2.aspx
Your ball bearings can take a lot of stress and abuse as they battle against friction, dirt, and physical forces. Flat surfaces on the balls will eventually develop when smaller particles get inside the bearing. This gives you a "gritty" feel, and can cause damage to other moving parts, such as spur gears that don't spin true on shafts that are supported by damaged bearings. Steel shielded bearings are more prone to getting dirty inside than Boca Bearings Green Seal or Ultra Seal bearings, which are maintenance free and lubed for life.

The first step to cleaning your bearings is to just soak them overnight in a bath of paint thinner or brake fluid. This helps to dissolve and break down the grease that the manufacturer packed the bearings with. Then allow the bearings to air dry or you can even blow them out with compressed air to remove all traces of excess lube. Now place a small drop of oil on the shield and give the bearing a spin. If the bearing still feels gritty, repeat the above steps.