I'd say:
Soothe Bell. Give it a Soothe Bell.
Massage it at a certain person's house.
Keep it in your party, but don't let it faint!
Rare Candies, Protiens, Calciums, Irons, etc.
Capture it in a Luxury Ball.
These 5 suggestions work for almost any 3rd-present generation game. If you want to evolve Pichu, Togepi, Igglybuff, Cleffa, Chansey, Azurill, Buneary, Eevee, Riolu, Munchlax, or (I think) Golbat, this is certainly the place to start (I think I got all the Pokemon that evolve with happiness).
the easiest way for me is to give it exp.share and face the elite 4
There are many ways to give it happiness. One basic way is to level the pokemon up. Then you can feed it proteins. You can also give it massages, and if you get the footprint ribbon on the pokemon it signifies max happiness
Give it a Soothe Bell, and walk around with it.
Give a Soothe Bell it raises happiness.
easiest way to train your Pokemon is by batteling each trainers or you can practice with wild Pokemon in the grass,cave,or sea
the easiest way; in hearthome city, talk to a fat hiker and he will give you an egg and when that hatches then it will be a happiny.
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the easiest way to level up your Pokemon to put it in daycare Pokemon will level up soon.Trade the Pokemon to friend and have it hold a lucky egg it will level up quickly.
trade it.
Buy them.
use the exp share
Either cheat or "chain" 40 pokemon.