The Destiny Knot
Prior to gen6, this item transfers only 3 ivs from 12 (6 from each Pokémon), and so the process is slower, more random. The basic principle is the same, though. Catch the appropriate Ditto with the correct nature (optional), give it a Destiny Knot (mandatory) and start breeding. Keep checking from the IV checker to ensure are you're getting all the perfect ivs in the newly bred Pokémon. Also, obligatory reminder to get a Pokémon with Magma Armour or Flame Body on your team to help with the hatching. Besides that, it's all luck. Keep trying!
You can get the item by talking to a woman on the Unity Pier in Castelia City.