These all raise happiness:
Pomeg Berry, Kelpsy Berry, Qualot Berry, Hondew Berry, Grepa Berry, and Tamato Berry (Emerald only)
Give Vitamin
Level up
Defeat strong Trainer (Gym Leaders, E4, and the Champion all count as "Strong Trainers")
Walk 256 steps (Every 256 steps, there's a 50% chance of the Tameness Value increasing)
These all lower happiness:
Faints
Bitter items (Energy Root, Revival Herb, etc)
Trading (resets it to neutral)
When your Pokemon is holding the Soothe Bell, all happiness changes are multiplied by 1.5.
All info was courtesey of Pokemon Forever, which was not forever after all. ;_;
Also remember that, in R/S/E, time is based on the clock you set in your room when you started the game, and Eevee evolves into Umbreon in the AM, and Espeon in the PM. Don't ask me why. That's how it is in R/S/E for some reason.