

There's a big focus on having Pokemon with the right Individual Values (IVs), Effort Values (EVs) and Natures.It's still geared toward Gen 5, as the community is still learning the ins and outs of X/Y, but most of the info there will stay relevant. Check out Smogon for different tactics, and check which Pokemon work well (and work well together).You really have to focus on Team cohesion a lot more. Try your hand at multiplayer: Link battles tend to be a lot different to Singleplayer battles - they have their own tactics, and the battles tend to be harder as they're more evenly matched (where Singleplayer is geared towards you winning).Smeargle works well for this - Teach it False Swipe, Mean Look, a Paralysing move such as Stun Spore, and something else (I usually run with a move to hit Ghost Pokemon) I usually set up a Pokemon geared toward catching other Pokemon.Trade between X & Y to get the ones you can't catch in your game.

Go back to different areas and catch Pokemon you're missing (check the Pokedex for their locations).The other answers are pretty good, but just to add a couple more points.
