These make Pokemon happy:
- Friendship berries, obtained in the Dream World
- Get a massage in castelia city
- Use the drinks you get from the vending machines
- Each 256 steps walked
- Levelling up
- The vitamins (this is my preferred method)
- Challenging gym leaders and E4 does in gen IV but idk about b/w. it's not a very efficient method either because the happiness raise will be negated if the pokemon you're trying to make happy faints
Holding a Soothe Bell increases the effects of the above, and catching the Pokemon in a Luxury Ball also has a small effect.
Don't use those herbs that say they're bitter tasting - those lower happiness, as does fainting.