remove hubs from spindle after each night of racing, clean the bearings good and lube them up, put back on and your done. replace when they no longer spin free or are worn out andmaking noise like the rear bearings
I like to take the rubber seals off the insides. I then drill a hole right by the mounting surface of the hub just big enough for the straw from my tri-flow can. then you just fill the hub up until it comes out the spindle hole and put the wheel back on and it covers the hole and you have an oil bath to lube your bearings
there is sometimes a metal cover in front of the rubber seals, you have to pop that metal seal off to get to the rubber seals. you can then pop out the rubber seals to expose the bearings and clean them, then put the rubber seals back in place. I take the metal covers off my bearings to make it easier to clean them, and just clean the bearings after every day of racing. But the post from jake17j about drilling a small hole in the hub sounds like a great idea to keep them lubed, but i wonder if that will let the dirt out of the bearings or not
some are hard to get out.. Some have metal backed seals some rubber. sometimes easier to replace a couple time per year. when you pull the seal it bends and is hard to replace . jakes method would work though it seems it would take a lot of tri flow to fill the hub.