My approach:
1) Take off the seals and pack bearings with marine grease. Put a bit more on the outer face of the seal before you fit the top hats
2) Spin your cups in gently. When they bottom out, is there even contact with the BB shell all round ?
a) If yes, tighten the cups in and fit yr cranks (do not overtighten the bearing preload).
b) If no, think about facing first. (I’ve never had this)
(it’s possible that the threads aren’t aligned right at either side, but you can come back to that possibility if you continue to ruin BBs despite having flush-fitting cups – I’m not aware of a way to properly correct that fault)
3) Ride bike, don’t jetwash and don’t leave for weeks between really wet rides – spin the cranks now & then if you are going to
4) If they start to sound rough, open up the seals & regrease. If they start to sound grindy, get some new and look at what you may have done wrong