-Give it a massage in Cyllage City.
-Use the Soothing Bag in Super Training.
-Give it a Colourful Shake, Rare Soda, Ultra Rare Soda, Perilous Soup, or EV Juices from the Juice Shoppe in Lumiose City.
-Use Friendship raising berries. (Grepa, Hondew, Kelpsy, Pomeg, Qualot or Tamato.)
-Level up in a battle.
-Battle against a Gym Leader, Elite Four member, or the Champion.
-Use a Vitamin. (HP Up, Protein, Iron, Calcium, Zinc, Carbos, PP Up, PP Max, or a Rare Candy.)
-Walking 128 steps (sometimes).
-Use a Battle Item (only works if it has less than 200 happiness)
Doing these things will lower happiness:
-Letting it faint.
-Using Energy Powder, Heal Powder, Energy Root, or a Revival Herb.
Hope I helped!