EV lowering berries work wonders, for not only making pokemon happy; But by getting rid of EV's you don't need as well.
Methods to improve friendship in this game uses the methods from the original Gold.... And maybe more I guess.
Leveling up,
Walking,
EV berries,
Battling a Gym Leader,
Massages
Grooming
Haircut brothers,
etc.
-So far the best method; Is a plain good old stroll with your pokemon. Take note that giving it a soothe bell, will speed up that process. Happy strolling.
For more info, and for stuff that lowers friend ship, just check out Bulbapedia and stuff.
Here