Quick Daycare levels
Put a character with "Rock Climb" against a rock wall. Tape down the directional pad in the proper direction so a character will continually walk up, then slide down, the rock wall. Plug the DS into the charger and leave the game turned on overnight. Each step the trainer takes in this cycle will increase the experience points of the Pokemon left in daycare.
Quick breeding
Pokemon left in the Daycare can be tricked into producing an egg faster. Traded Pokemon will breed faster than caught Pokemon, and two Pokemon of the same species will produce an egg faster than Pokemon of two different species.
Ditto breeding
Pair a rare or starter Pokemon with a Ditto to breed more of that Pokemon. Catch a Ditto on Routes 34, 35, and 47, in the Cerulean Cave, or in the Safari Zone. Leave the rare Pokemon with the Ditto in the daycare until they produce an egg. This will allow a player to breed a rare Pokemon to trade and keep his own copy. This will not work with Legendary Pokemon.
Incubating eggs
Keep a Fire-type Pokemon with either the Flame Body or Magma Armor in your party to cut in half the number of steps the trainer needs to take to hatch the egg.