There is a glitch that enables you to transfer unlimited amounts of Pokés in only one day, discovered by PKMN.NET:
First, you'll need two Pokémon Advance GamePaks.
Insert the first GamePak and transfer 6 Pokés, and catch'em in the PalPark, just as usual.
Then, shut down the game(don't SoftReset), and swap the GamePak with another, and power up the DS.
On the menu choose Options and set the date to one day forward, and start the game.
Now, choose Migrate, and it'll tell you to sync the Game Clock to the DS clock, choose yes, wait a while for it to save, and shut down the game.
Swap back to the first GP again and you'll see that you can once again transfer 6 Pokés; keep doing that and you'll be done real soon!
I used that to transfer my complete RubyDex, around 250 Pokés in only one day!