Contrary to popular belief, putting a Pokemon in the PC doesn't decrease happiness(would that really be fair if it did decrease happiness? You can only hold six Pokemon). A Soothe Bell increases the happiness of the Pokemon as you walk around if it is in your party. Berries and vitamins are also great ways to speed up the process if you have a little extra moola lying around. A luxury ball and friendship ball also work similarly to the Soothe Bell if the Pokemon is caught in one. Trading a Pokemon resets it's happiness to its original amount, feeding it bitter items, including bitter Poke Snacks, also lowers it happiness. Fainting is the only other thing that lowers happiness, so avoid doing those things, but don't worry about the PC. Hope this helps!