Well, I'm just going to assume that a) you've completed the game and b) you've gotten all the O-Powers. If you haven't completed the game, disregard this entirely. If you don't have all the O-Powers, look here.
So a good way to start out is to get some Lucky Eggs. If you're lucky enough to be friends with someone who has a Chansey Friend Safari, you can find Lucky Eggs on wild Chansey. If you aren't friends with such a person, you can probably get some by asking around on Showdown! or other such websites.
Next, you want to pick your location. The best training spots in the postgame, for me, are Victory Road, the Pokemon League, and against Serena/Calem in Kiloude City. The Lumiose City restaurants Restaurant Le Wow and Sushi High Roller are good sources of training too, but they do cost a lot of money.
Hope I helped! :D