#6 Forgo the Exp. Share
The Exp. Share, available in every main-series Pokemon game, received an upgrade (or downgrade, depending on how you look at it) in Pokemon X and Y which makes the game the easiest Pokemon yet. Before X and Y, the Exp. Share would dish out experience points to a Pokemon who did not participate in battle only if that Pokemon was holding the item itself. Now, however, the Exp. Share awards experience to your entire team of Pokemon. If the Exp. Share remains on for the course of the game, your Pokemon will far exceed the level of every trainer you meet.
However tempting it may be to use it, turning off that Exp. Share as soon as you get it will assure your adventure through X and Y's Kalos region is much more challenging. Your team will always be slightly lower level than the trainers you meet, so creating a diverse team of Pokemon to counter most situations becomes more of a necessity.