Walk or ride your bike around the game world with the Pokémon in your party. As you travel, your Pokémon's friendship level will increase slightly.
"Pokémon Yellow" introduced friendship as a game mechanic to the Pokémon franchise, and each successive generation of games has utilized Pokémon friendship in some way. As you battle other trainers and gain experience, your friendship with your Pokémon will grow stronger, but each game offers several other ways to strengthen the bond between you and your Pokémon. Each Pokémon in your party has its own friendship level.
Feed your Pokémon vitamins or Pomeg, Kelpsy, Qualot, Hondew, Grepa or Tamato Berries. Each berry or vitamin you feed your Pokémon will boost its friendship level, but it may also affect your Pokémon's stats.
Place your Pokémon at the head of your party, and then talk to special people in the game world. In "Pokémon Black" and "Pokémon White," talk to the masseuse in Castelia City. In "Pokémon HeartGold" and "Pokémon SoulSilver," talk to one of the brothers in Goldenrod City, who will give your Pokémon a haircut. In "Pokémon Diamond," "Pokémon Pearl" and "Pokémon Platinum," get your Pokémon a massage from a lady in Veilstone City. Generally, you can only receive one massage or haircut from each person each day.