Why clean a bearing?
Often bearings will collect lube, bits of string, dirt, hair, and whatever else can find its way into the gap of a yo-yo. Over time, this accumulation of debris will start to decrease sleep times and detract from the performance of the yo-yo. When this starts to happen, it is time to clean the bearing. Cleaning the bearing well will remove the unwanted debris, as well as dry the bearing of any lube you put on it.

What is the best cleaner?
Many people use many different things to clean their bearings. The most common is Isopropyl Alcohol; another is lighter fluid. Both of these are inexpensive, come in large quantities, and you should be able to find them around the house. Other people may use nail polish remover, Goo Gone, or even sonic jewelry cleaners.

How do I do it?
The easiest way is to pour a little bit of alcohol into a small container, and let it soak for a while. Shake it a couple times, to get the stuff out, then take it out. It should be pretty clean, although it takes a couple cycles to get it completely clean. NOTE: If you use Isopropyl Alcohol of a low concentration, such as 70%, it will rust your bearing after a while. Keep an eye on it, and don't leave it in there too long.

What good does it do?
Cleaning a bearing cleans out all the junk that has accumulated - but it also cleans out the lube. Thus a yo-yo will sleep longer but not respond as well after having its bearing cleaned. If it needs lube to respond, then you will need to lube it again afterward. Also, really clean bearings tend to make noise and "scream" rather loudly. Part of the "scream" is the balls slipping around the race, which is not a good thing. If this bothers you, then lube it - the noise will go away and that will be fixed.