Another thing I have found helps bearing life in BBs is to totally clean out the whole BB, really scrub it down with degreaser and wash it out, including the crankshaft and all the retainers/etc., till they're shiny new looking.
Get high-temperature grease like for car wheel bearings, differentials or similar; I use a tube of red grease called "Mystik JT-6", by CITGO. I know what I have is old so I don't know if the identical one is still available anywhere, but something similar should be. It's listed as an NLGI #2 grease, for wheel bearings, chassis, etc. It's "dropping point" is listed as 550F, so it *never* gets gooey or runs or leaks on any bicycle stuff I've used it on.
It's way overkill, but it works.
Coat a paper towel with the grease, and stuff it up into the seat tube (from inside the BB) a little ways, basically to plug the tube from leaking anything down into the BB. Do the same with the chainstays and the downtube (or any tubes connecting to the BB). If you like, you can also do this at the other ends of those same tubes, to prevent anything from getting inside them in the first place.
Fill the whole BB with the grease, as you install the bearings (packed with the grease) and crankshaft (coated with the grease), so you have essentially a solid BB with nowhere for dust, dirt, or mud or water to get in there to deposit stuff onto the bearings in the first place. Then the only wear will be from stress grinding away bearing or race surfaces, and leaving metal deposits in the grease. Can't change that.
I do the same thing with my wheel axles/hubs.
It adds some friction, but it keeps wear to a minimum, and given the flinty dirt we have around here in Phoenix, keeping the dirt out of the bearings is necessary.