Pop the bearing out of the pulley:

[Linked Image]

[Linked Image]

Then use a small knife to pry one of the seals out of the bearing (carefully, don't damage the seal, you re-use it), then clean and grease it:

[Linked Image]

I try to re-lube mine every time I change belts or otherwise have that part of the engine torn down. Or I carry some of the common size bearings if you can't fix it.