How do I rejuvinate ball bearings?

If they are ceramic, forget it. They are toast.
If they are steel you can soak them in kerosene to get rid of the bits and old grease and then repack with new grease . If you don’t have kerosene you can spray with carb cleaner.
(The link I included is for cars but it is still true for longboards.)

For some reason this question makes me giggle. Anyway, use the same kind of degreaser and lube that is used for bike chains. Follow the directions on the can :)

I always use WD-40 to clean them. Then I add the lube I used for my bike chain. Seems to work pretty well. And I have been skateboarding for over 20 years.

Thanks for the input guys!

@johnpowell Are you familiar with ABEC 5 Greaseballs? This is what I have.

From what research I can gather, these have a clip that I need to pry off on one side to grease them up. But I guess once I get one side off, I just leave it off? Is this right?

I’m not familiar with those bearings. But you will want to apply a little lube and put the cover back on. You want to keep the dirt and dust out. They do make open faced bearings. But those are mostly used for half-pipes. Since half-pipes are a cleaner environment.