Instructions
Catch your Pokemon with either Friend Balls or Luxury Balls. Friend Balls, which raise your Pokemon's Friendship Level quicker, are purchased from Kurt in Azalea Town. Luxury Balls increase your Pokemon's happiness. You can buy them at Goldenrod Department Store.
Put the desired Pokemon at the front of your team. Whenever you enter a battle, that Pokemon comes out to fight first. Pokemon are happy whenever they can leave their Poke Ball.
Give items to Pokemon to make them happier and friendlier. Use stat-boosters like Protein and Zinc to greatly increase happiness. Wild berries also work. Potions and other store-bought healing items increase friendliness, too.
Get your Pokemon groomed. In the Goldenrod Tunnel, two brothers offer haircuts for Pokemon at differing prices. The more expensive haircut is typically better. If you don't have money for grooming, take the Pokemon to Pallet Town between 3 and 4 p.m. Go to Blue's house and speak with Daisy, his sister, and you'll get a free haircut.
Put your Pokemon in the Pokewalker, an electronic device that came bundled with the game. Select "Connect to Pokewalker." Select a Pokemon from the list to send to the device. Take the Pokewalker with you wherever possible. Each step you physically take with a Pokemon in the device makes that Pokemon happier.