Eh, you just surround yourself with four-leaf clovers and rabbits feet. And nail up those horseshoes (with the ends pointing up so the luck doesn't drain out).
I mean, it's not like luck is a real thing, so naturally, there's no real way for a fictional thing to increase.
But, in the real world, it's not like a grown adult tends to actually be able to get smarter. A person might learn more things, but they're not going to become more intelligent. Realistically, people's stats don't make much in the way of changes after a certain point. If you're not the sort of person to exercise, you are unlikely to become such a person. And realistically, there's often a trade-off. If you're exercising in your free time, then you're not reading as much as you used to, so your intelligence should go down. Or if you're a muscle-bound hero who starts reading a lot, then you're not exercising as much, so your strength should go down.
But I haven't see any game systems where your stats can decrease from non-use.
So, if there's any argument about realism regarding ability scores in any game system, I would find that to be very strange.