Verified answer

give them calcium, zinc, ect.

or go to the massage lady in veilstone city (just west of the pokemon center and through the rocks *there is a thin path*)

I'm not 100% positive about this one, but I think walking them in amity park works, too, but you can only walk certain pokemon.

But none of those work as well as raising it's level. perhaps you should find some heart scales underground if you want your pokemon to learn attacks it missed.

mmmmm......I don't really know!