The best way is to remove the shields, clean out the grease and gunk, lube, and re assemble. Some people use an ultrasonic jewlery cleaner with some solvent, others use the RPM bearing blaster with motor spray. I just use motor spray with the straw and blast it out. For lubrication, I used to use hi temp moly grease, but have switched to synthetic motor oil.
The trickiest part of all this is removing the shields. If you damage them too much, they wont go back in. Rubber, you poke with a sharp probe and pull out. Metal is a little trickier. Most metal sheilds are held in with a very very small c-ring. I use a sharp exacto blade to remove it. I have also had press fit metal too and had to pry it out. Bent it up pretty badly too, but I was able to flatten it back down and reinstall it. I figured I had nothing to loose, since it had seized up at a race and I didn't have any preplacements. I got it working like new. Its still in my car.