Instructions
Capture all new wild Pokemon while on your journey to beat every gym leader, the Elite Four and Champion. By the time you accomplish all three goals, you will have 100 to 200 Pokemon in your Pokedex. Don't use repels unless you have caught every Pokemon in the area.
Trade with your friends often, especially if you have friends who own the sister version of your game. Each game version has Pokemon exclusive to it. For example, Pokemon Ruby has Groudon as a legendary Pokemon, and Sapphire has Kyogre. The only way to get exclusive Pokemon from a version you don't have is to trade.
Migrate Pokemon from GBA Pokemon games using Pal Park. In the GBA Pokemon game, place the six Pokemon you want to migrate into your PC. Insert the GBA cartridge into your DS. Go to Pal Park in the DS game. Look in each area for all six Pokemon. You can migrate only six Pokemon every 24 hours.
Evolve your Pokemon into any forms that don't appear in the wild. For example, In Pokemon Black, you can catch Riolu but not Lucario in the wild. To get Lucario in your Pokedex, evolve your Riolu once it has gained maximum happiness.
Get the Explorer Kit from the Underground Man in Eterna City. The Underground Man is in a house next to a Pokemon Center. Place the Explorer Kit in your Key Items Pocket and talk to the Underground Man. Use the Pickaxe or Hammer to look for fossils. Take the fossils to the Museum to revive them into ancient Pokemon.