Instructions
Take your Pokémon for a walk. Walking 256 steps with your Pokémon increases his happiness rating.
Get your Pokémon's hair cut by the haircut brothers, who sometimes wander the Goldenrod Tunnel.
Visit Daisy Oak in Pallet Town between the hours of 3 pm and 4 pm. She will offer to groom your Pokémon.
Level your Pokémon up using either an item or by having him fight other creatures. Every additional level results in a happiness boost.
Feed your Pokémon EV berries for a massive happiness increase. Kelpsy, Tamato, Qualot, Hondew, Grepa, and Pomeg berries are found scattered around the world of "HeartGold."