Keep the Pokemon in your party
Walk around with the Pokemon in your party
Level up the Pokemon
Use the Pokemon in battles
Give the Pokemon vitamins
Give the Pokemon friendship-raising berries
Give the Pokemon a Soothe Bell
Avoid letting the Pokemon faint
Heal the Pokemon at Pokemon Centers
Use grooming or massage services when available
Camp, picnic, or play with the Pokemon when available
Teach the Pokemon new moves with TMs or move tutors
Keep the Pokemon from being boxed for long periods