-Capture Pokemon with Friend Ball: Very effective. The best possible way, your Pokemon will start out with a lot of happiness. It will be very easy to gain enough happiness for you Pokemon if you use a Friend Ball to capture the Pokemon.
-Trading Pokemon: If you trade a Pokemon, it's happiness drops to "normal".
-Walking/Biking (Having it in your party): Helps a little, not much
-Using a stat-up item: It helps, but not a lot.
-Pokemon gains a level: Quite effective.
-Haircut Brothers at Golenrod City's Underground Path: The amount of happiness each time is random.
-Daisy's Grooming at Pallet Town 3:00PM - 4:00PM: Simular effectiveness as Haircut brothers.
Pokemon Faints: Decreases a little.
Heal Powder/Energy Powder: Decreases a good amount, 5 times as much as fainting.
Energy Root: Decreases a lot.
Revival Herb: Decreases happiness the most.
Rumors that are NOT TRUE
Increase (False): Win battle against a trainer, Gym Leader, Elite 4, etc.
Decrease (False): Pokemon status, Poison, Paralyze, etc.
Decrease (False): Storing Pokemon in box.