Instructions
Catch your Pokemon in a Luxury Ball. Pokemon that live in Luxury Balls gain happiness at double the rate of Pokemon that live in other Pokeballs.
Get a Soothe Bell item from the National Park area in the west of the Johto region and give it to your Pokemon to hold. This will double the rate at which that Pokemon's Happiness increases.
Assign the Pokemon to your traveling party of six Pokemon. Every time you walk 256 steps, each Pokemon in your party gains one Happiness point.
Use your Pokemon in battles so that it gains experience points and levels up. The number of Happiness points your Pokemon gains from leveling up depends on its current Happiness score. If it is below 100, it will gain five points; from 100 to 200, three points; and from 200 up, two points.
Feed your Pokemon Vitamins such as HP Up, Carbos, Iron, Protein, Calcium and Zinc. Depending on the Pokemon's current Happiness level, each of these gives a boost in the same manner as leveling up.
Go to Goldenrod City and take your Pokemon to the Haircut Brothers for a haircut. This will boost the Pokemon's Happiness by between one and ten points depending on its reaction to the haircut.