If they are the metal shielded type I just use some electric motor cleaner or nitro wash and give them a good blast over a rag to get any dirt out of them, then I wipe them clean and apply some bearing oil, rotate them to work in the new oil and wipe off any excess oil with a clean rag and they are ready to go. As for the ruber sealed type unless the seals are worn out all you should need to do is put a little cleaner on a rag and wipe them down, if the seals are worn then you could treat them the same way as the metal shielded, at least thats what I do...