Breeding Pokémon has had an added twist added in Emerald. Firstly is Passing on of natures which can now be done by attaching an item to the Female Pokémon or the Ditto...this item is the everstone so you need the following to do it:

Female Pokémon

When you do this with both a female and a Ditto, your Pokémon has a 50% chance of inheriting the nature from either Pokémon.

But thats not all, there is an added bonus of having certain Pokémon on your team which have specific Abilities means the steps required to hatch the egg is cut in half. Say for example you have a Pichu egg, it would normally take 2,560 steps to hatch...but now it'ld take 1,280 Steps to hatch. Having the following Pokémon on your team will cause this to occur:

Ability: Magma Armor
#218 Slugma
#219 Magcargo
#323 Camerupt
Ability: Flame Body
#126 Magmar
#218 Slugma
#219 Magcargo
#240 Magby