Go back in time and go to a Nintendo Event when they still existed, or get a Gameshark or Action Replay.
Get Lugia and Ho-Oh on Navel Rock (8 Island), Latios, Latias, and Blaziken from Ruby, Sapphire, or Emerald, and get Jirachi through cheating devices as mentioned before or get the bonus disk from Pok�mon Colosseum and get Jirachi from it.
Otherwise, battle the Elite 4, train more, blah, blah, blah...