in my metal shielded bearings I use dry lube. To be specific I use this stuff http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXATCH&P=7
I found that the bearings spin a lot better with the dry lube and don't get as much dirt in them. Rubber sealed bearings I just buy new ones when they don't perform the best anymore.