Originally Posted by mprince

Servicing the bearings while installed really isn't too bad -

1. remove snap ring and neoprene seal
2. I use spray degreaser to clean (King advises against citrus based degreaser as it interacts with their anodizing) - automotive brake cleaner works fine for me.
3. I spray the bearing with compressed air to remove residual degreaser
4. Pack the bearing with just about any grease of your choice
5. Replace neoprene seal and snap ring - cup done

I have my cups out of my frame right now as it is at the painter for a facelift. I serviced the bearing at my desk (with a detour to the kitchen sink to degrease with hot water and dish detergent/toothbrush and a trip to the garage to blow out the bearings with my air compressor) using a tube of some old Pedro's grease from my toolbox. Total time was well under 30 minutes, this headset is 21 years old and has been serviced ~3 times - has been clean as a whistle every time.

If you have a repair stand it's not too hard to do with the cups still on the bike.