Pokémon Sapphire Cheats

Lots of berries
At the start of the game, set your clock to the time it is, but swap am & pm (eg. if it's 10:00am put 10:00pm)
Then when you get far enough to visit the berry master's house, go there. Take all the berries and plant new ones. Because it is night when you play during the day, it will rain. (It always rains at the berry master's house during night time.) Thus the game will treat it as if you're always watering the berries whenever you play.
So the berries will grow faster and with more berries.