Treat your Pokemon with love to increase friendship points. Never let them die and always use a potion if they are injured. Also give them special treats such asProtein, Iron, Carbos, etc. Never give them bitter healing items. Placing them in your PC Boxes for a long time or in the Day Care doesn't affect the friendship points of your Pokemon, nor does leveling them up with Rare Candies. Use the Poketch's app Friendship Checker you get at the Eterna Pokemon Center to monitor your Pokemon's friendship level.