Easy Hatching (Exploit)
In Lumiose City, there is a very convenient circular walkway surrounding the big pokemon building (it's yellow on your map, and resembles the Eiffel Tower). The path serves as a very nice endless road, perfect for hatching your new babies. Find a spot where you won’t bump into anything and start marching! You can place a quarter or some other coin to hold the circle pad slider in place. Your trainer will roam around in circles and you can leave to do something else. When you return to your system, you should have a new level one hatchling.
Infinite Heart Scales
Heart Scales can be used to teach your Pokemon moves you forgot or never learned at the Move Relearner in Dendemille Town. To obtain Heart Scales in Pokemon X and Y you need the Old Rod from Ambrett Town in the Aquarium by the Magikarp Statue and TM46, Thief, from Cassius's house in Camphier Town.
Use the Old Rod on the Route 8 - Muraille Coast to battle Luvdics who have a 50% chance of carrying a Heart Scale. Keep fishing for an infinite supply of Heart Scales. Switch spots if Luvdisc doesn't take the bait.
Talk to the man in the Pokemon Center in Shalour City to hear him tell you about a game he is playing where a cool little dude "runs around and whacks enemies to the beat of the music." That is an apparent reference to another game from the game's developer called HarmoKnight.
Float like a butterfly, sting like a bee!
Sometime when training on the punching bags in super training, you will be told to "Float like a Butterfree, sting like a Beedrill," a sly reference to Muhammad Ali's famous quote.
On your birthday, enter any pokemon center and let the party begin!
3DS XL and Wii U Easter Eggs
Upon waking up in your house at the beginning of the game, your character will be holding a Pikachu 3DS XL. On the ground is a Wii U. If you are a girl it will be white, and black if you are a boy.
Gym Battle Preparation
If you find yourself having too much trouble facing off your current Gym Leader, have no fear. Before challenging the Gym Leader, you can walk, skate, or bike toward the next route. The trainers you battle there will help to increase your team's overall level. Some routes will be blocked, but others are wide open and allow you to roam ahead of time, giving you the opportunity to level up a fair bit before you return and challenge that pesky Gym Leader.
Does the day care man have an egg for you?
Unlike the previous two games were the day care man would call you over if he had a egg for you, the caretaker in X and Y doesn't call your attention to the fact. You have to check, and he may or may not have anything for you. However, there's a way to tell without heading over to engage him in conversation. If he faces the road, that means he has an egg. Otherwise, he'll be looking toward the building. Knowing that fact can save you some hassle.
Cafe Pokedex Help
There are three cafes around Kalos that you have to pay to enter. Each possible seat you choose will allow you to talk to patrons who will show you Pokemon for your Pokedex. The 100 price option will show you one Pokemon. The 1,000 price option will show you two Pokemon. The 5,000 price option will only show one Pokemon, but it will be guaranteed to be one you haven't seen. You can only do each one once, so use it only as a last resort for pokemon you can't find.
Secret Background Battle Items
Occasionally in battle, you will notice various things in the background, ranging from trees in the field to rock formations in caves. These seemingly innocuous background objects can be destroyed and release an item.
It seems that the various objects react to specific ranged attacks. While Surf may work on one, it won't work on another, and other moves such as Earthquake will not effect the same object Surf does.
Some of the known moves that work are Air Cutter, Twister, Razor Leaf, Pedal Blizzard, Surf, Rock Slide, and Heat Wave.
Earn Easy Money
The Trainers at the Battle Chateau pay out amounts in excess of P1,000 per battle when you start out, but as you gain rank these amounts skyrocket to north of P20,000 per battle. Combining this with an Amulet Coin and/or the Prize Money O-Power can result in a very profitable day at the Battle Chateau.