Verified answer

level them up (without exp share)

give them berries that up friendship

give them a soothe bell and walk around

feed it poffins it likes

There are many different ways. 1. Battle with it 2. Don't let it faint 3. Feed it status items 4. Go to Amity Square (this only applies to Clefairy, Drifloon, Pachirisu, Psyduck, Pikachu, Buneary, and Happiny.) 5. Visit a girl in Veilstone City and she'll give one of them a massage once a day. 6. Give it berries that up its friendship (however these lower it's stats) 7. Give it a Soothe Bell. 8. Simply walk around or do anything with it in your team

Go to Amity Square

you battle with them a lot without letting them faint