Every time I put new bearings in I make sure there is sufficient grease in them.
First mistake! Once the seals are snapped into place its almost impossible to remove them without nicking the sealing lip, instant leakage path. If you buy a decent bearing, they should have enough grease in out of the packet.
Are you fitting them with the right tools? If you fit them with the wrong tools its possible to move the races in relationship to each other, and damage the seal (plus the brinnelling of the races!). You can also put them in skewed/off axis, this either accelerates wear or can ovalise the bearing seat (game over for the hub, every bearing you fit from here on in will wear prematurely.)
Also, are your axles straight?
It's not necessarily a sealing issue either, tiny bits of ground up bearing will also leave the bearing gritty, brown and not turning very well.
(FWIW, i'm still on original bearings on some Pro2s which i've done three years on. I usually get 2-3 years out of a set of hope bearings)