If you have harvested a lot of EV reducing berries, the fastest method to get the happiness needed to evolve that I know is feeding to that Pokemon between 18-25 berries, while it's holding a Soothe Bell.
You can check when a pokemon gets enough happiness to evolve when the 2 small hearts appears when touching the pokemon in the poketch application.
If you don't have too many berries, you can use the massages at Veilstone, winning battles with that pokemon, or even walk with that pokemon in the party, but keep in mind that Soothe bell will double the amount of happiness points obtained.