Sitrus Berries
Once you get surf, go to the berry masters garden and pick all the berries.
Plant all the sitrus berries you have or get from now on in the garden and go back to water them.
Keep on doing this untill you get to the pokemon leauge. The berries save you from having to buy potions.
Also, when you get to Stephens Metagross, use Blazkin's (if you have it) over heat or just use a pokemon with over heat like Groudon.
More tricks, tips and cheats for this game are right here - Pokemon Ruby cheats
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