Pokémon Global Link, in combination with bulk Wonder Trading to rack up easy Poké Miles, is a pretty great way of building up PP Up. Trading off the rejects from a breeding project generally earns enough miles to max out the PP of the Pokémon you bred too, unless you got especially lucky while breeding.I feel like I've mentioned this before, but I hate how you can't get easy PP Ups and Maxes in this Generation. Last gen, it was as easy as camping in front of that one rock, grinding Star Pieces, then getting seven of them from that one guy in that town you can only get to through the Battle Subway. That and you often got Ups and Maxes from people at the Stadiums in Nimbasa. I miss that so much.
Wonder Trading also helps with your odds of winning a PP Up / PP Max in the Loto, though that's limited to one per day.