This is a straightforward demo of how to disassemble, clean, grease, and re-assemble a skateboard bearing. Just in case you wanted to see it done. It's actually terribly easy if you've done it once.
Step 1: What You Need
Acetone or other solvent. (97% Isopropyl Alcohol is good too.)
A bowl or cup to put it in.
A small pin or straightened staple to remove the bearing shields.
Step 2: Pop the Seals
The seals can be popped out from the other side with a small pin or straightened-out staple. Stick your pin between the ball cage and the outside bearing race and press the seal out. Aim for the center of the seal, because it is made of metal and you won't puncture it.
If you've got metal shields, they're held in place by a circlip around the outside edge. Get your pin under one edge of the circlip and it'll come off easily. Then the metal shield just pops out. See this website for pictures of how to get the clips off.
Step 3: Soak and Clean
Alternately tap them on the bottom of the bowl and spin the bearings in your hand until they feel grit-free. Once you get a bearing cleaned, spin it dry and place it on a clean paper towel (balls down) to dry.
Repeat for the other 7 bearings.
While the bearings are finishing drying off, clean up the seals in the acetone. Be gentle -- you don't want to bend the inner metal ring.
Step 4: Optional: Filter and Save Your Acetone
If your coffee filters are like mine, it won't be perfect. But it'll be good enough to re-use a few times, or you can experiment with multiple filters. Store your used acetone in a well-marked (skull and crossbones are good) airtight jar wherever you keep your deadly chemicals.
Step 5: Grease and Re-seal
Bike grease is fine. Marine grease will work. Silicone or lithium grease? Sure. Molybdenum grease? Overkill, but if you've got some lying around the house, you can use it.
Spread the grease around with your finger and roll the bearing back and forth until you can see all the balls adequately covered.
Gently place the seal back on. It will "snap" into place. Then, since you completely un-lubed it, run your greasy finger around the outside of the rubber seal just enough to keep it moistened. Also get some grease on the metal surfaces to help keep the rust away.
Note at this stage you could be using an oil instead of a grease. Good oils are Bones Speed Cream, Tri-flow, and other similar lubricating oils. Motor oil will work too. Bad oils include WD-40 and 3-in-1 (too light). If you use oil, 2 drops are usually enough. Spread it around by spinning the bearing and re-assemble.
Step 6: You're Done!
Grease is nice because it needs maintenance less often than oil. On the other hand, it probably spins a little slower. Still, a clean and lubed bearing spins much much better than a dirty one, so it's good for you in the long run.
Note that you can switch back to oil when it finally stops raining and all the darn salt is off the freaking roads....
How often should you do this? It depends on how much you ride, and how often in crappy conditions. For greased bearings, try once every month at first or more often if you're in the wet a lot.
Oiled bearings need maintenance every couple weeks if you ride every day. I tend to clean them out right after a rain ride -- the oil just doesn't provide as much rust protection.