Guide to Effort Values in Generation VI

Effort Values or EVs are gradually added to each stat as a reward for pokémon battling experience. Each pokémon you battle either in the wild or by battling a trainer or Gym leader in the game has an assigned Effort Value. Unlike experience points that are divided among all the pokémon in the battle and those with Experience Share, all the pokémon involved receive full share in EVs.
These Effort Values range in each stat from 0-252. The maximum total Effort Values counted towards each pokémon is 510 for all stats combined. For every 4 EVs in one particular stat, 1 point is added to that base stat. Since you need 4 EVs to add one stat point, the maximum EVs that can be used is 252 for each stat equal to 63 stat points. Similar to IVs these will be distributed as the pokemon grows in level until it reaches level 100.
Effort Values are still maxed out at 510 in this generation, but the cap for each category has been set at 252 preventing training up to the previous 255 and gaining unusable effort points. The Super Training screen on the 3DS will give you a visual tracking of your EV training and when the bar to the right has been filled, a biceped icon will appear above the training bag indicating that your Effort Values have reached their max. There is also a man at the Pokémon Fan Club in Laverre City that can tell you if you have reached your goal and award you with an Effort Ribbon.
In previous generations, Effort Values were awarded with the next level up making it impossible to add them through battling once a pokémon reached level 100. Now they are added after each battle
One neat trick if you want to know exactly where your training stands is to save the game and use a reset bag. Your current EVs will be displayed numerically before they disappear. Just soft-reset without saving after doing this.

While the training items have remained the same since the past generation, the Experience Share has changed. Previously a pokémon had to appear in a battle for these items to take effect as pokémon could either hold Experience Share or a training item. With the new Experience Share not being a hold item, pokémon no longer have to appear in the battle to receive the Effort Value boost and multiple pokémon can be trained at once. Also capturing pokémon now adds experience points and Effort Values to the members of your party.

Macho Brace

Macho brace is a hold item that doubles EVs received from battling.

Power Items
There is a power item for each stat. They are hold items but reduce the pokémon speed one stage (50%), but increase the EV gain by 4 points for every pokémon that it battles.

• Power Weight - Increases HP EVs gained
• Power Bracer - Increases Att EVs gained
• Power Belt - Increases Def EVs gained
• Power Lens - Increases Sp Att EVs gained
• Power Band - Increases Sp Def EVs gained
• Power Anklet -Increases Speed EVs gained

Pokérus

Pokérus is a virus that doubles the EVs received from battles, this takes into effect any Power Item or Macho Brace boost first and doubles that amount.
It can be found by battling infected pokémon in the wild. The odds were 1 in 21,845, but those may have doubled as I have caught it in two of my games.
Pokérus can spread among the member of your party. When they are affected a purple box will appear to the right of their typing in that pokémon’s summary. When they are cured a smiley face will appear to left of their Kalos pentagon. Once they have been cured they cannot re-infected other pokémon. Curing usually happens from leaving the infected pokémon in your party when the game clock passes midnight. A safe way of keeping Pokérus intact is to leave them in your PC.

Sometimes you get unwanted Effort Values either through gameplay, leveling up, or you just change your mind on where you want your EVs. There are EV reducing berries that can be created by mutating berries found in Kalos. They will increase the pokémon’s happiness while reducing their Effort Values by 10 points. When used your pokémon will either become more friendly or it will state that it adores you if your pokémon is at max happiness and your base stat will be lowered until it says it can’t go any lower. If your pokémon is at max happiness and its base stat is at its lowest than it will say it will have no effect. These berries are:

• Pomeg Berry: - HP
• Kelpsy Berry: - Attack
• Qualot Berry: - Defense
• Hondew Berry: - Sp. Attack
• Grepa Berry: - Sp. Defense
• Tamato Berry: - Speed

Two new methods of EV reducing have been added in this generation that will eliminate all the EV Training and set them back to zero.

Reset Bag - One of the training bag found in Super Training. Hit it 100 times and all your past mistakes will be forgotten
Perilous Soup - In the Juice Shoppe in Lumiose City have the girl on the right make a juice with a Kee Berry and Maranga Berry. Whatever pokémon drinks this will have a fresh start at EV training.

For more info on how to get these berries check out:
Kalos Berry Fields

Vitamins will increase a pokémon’s Effort Value by 10 points, but cannot raise it above 100 (or the total EVs above 510) so if you want to use them, do it early in the training process. These Vitamins can be found through gameplay, received over pokémon Global Link, bought for $9,800 at the Laverre City pokémon center or exchanged for 2 BP at the Battle Maison.

• HP Up raises HP
• Protein raises Attack
• Iron raises Defense
• Calcium raises Special Attack
• Zinc raises Special Defense
• Carbos raises Speed

Wings increase Effort Values by one point and are more useful for fine tuning stats rather than growing them. Wings can be given as prizes in Super Training and at the pokémon Global Link. A Health Wing can be obtained every day in Ambrette Town by having a pokémon that has a high enough speed to please the lady in the house near the pokémon Center.

• Health Wing raises HP
• Muscle Wing raises Attack
• Resist Wing raises Defense
• Genius Wing raises Special Attack
• Clever Wing raises Special Defense
• Swift Wing raises Speed

Juice

On Autumnal Avenue in Lumiose City there is a Juice Shoppe where you can purchase and create Berry Juices for your pokémon to drink. You can chose which pokémon in your party you wish to give the juice to after it is bought or created. On the left you can purchase pre-made juices of various strengths and on the right you can create you own juice once per day. These juices can raise Effort Values from 4 to 32 points depending on the berries you use.

This thread talks about he Juice Shoppe

Super Training is a new feature in Pokémon X and Y. There are two methods of EV training your pokémon here.

Core Training
Core training is done using the training bags by tapping the icon in the bottom right of the screen. Small Bags provide only 1 EV point to a specific stat. Medium bags give that pokémon 4 EV points and Large Bags give it 12 EV points. They are easy to use, simply tap the bag with the stylus repeatedly. Bags are obtained by participating in the Super Training Regimen or sometimes hitting an empty bag when an open spot is available. Keep in mind that you can only keep 12 training bags and any after that you will not have an option to keep any more.

Super Training Regimen
There are three levels of the Super Training regimen. You can access these by tapping the icon on the bottom left of your screen. Super Training level 1 awards the pokémon 4 EV points if you complete the training before time runs out regardless of whether you are awarded a ribbon or not. Once a pokémon has completed all the level 1 Super Training then level 2 will be unlocked for all your pokémon. Level 2 will award your pokémon 8 EV points each time you complete the training exercise. Once a pokémon finishes all the level 2 exercises then Level 3 is unlocked for all pokémon. Level 3 awards pokémon with 12 EV points upon completion of each Super Training exercise. Pokémon can use level 3 training right away, but it is very difficult. As the pokémon's EVs grow, its hits become more powerful and the danmage it takes is lower.
There is also a Double-up training bag that when used before the Super Training regimen that will double the EVs from training. While pokérus does not double EVs gained from Super Training it does raise the likelihood of encountering these Double-up bags.

Several new and different methods of EV training are available in this generation.

Let’s start with the old fashioned way. Find a spot where pokémon of a certain EV are common and go to work fainting them. While the large number of pokémon found in most routes makes this more difficult, there are still areas that are effective to use this method. For example surfing in Azure Bay you encounter mostly Tentacools and Mantykes each providing you with Special Defense EVs. Walking in Terminus Cave provides mostly Defense EVs especially in pokémon X. Magneton and Litwick are common in the Lost Hotel and provide you with Special Attack EVs. Zubats and Meditites in the Connecting Cave provide Speed EVs as well as Delibird and Sneasel on Route 17.

Horde Training
A new and the most productive way to EV train is with hordes. While standing in a cave or grassy patch and using Honey or a pokémon with the move Sweet Scent, five pokémon will appear at once. Combined with a Power Item, a pokémon can gain 25-30 EV points from each battle (50-60 with Pokérus in effect).

Chain Fishing
One of my favorite ways is by chain fishing. While the better rods give you a 2-3 different fish, the Old Rod only gives you one type and no matter where you use it that pokémon will give you 1 Speed EV. On Routes 15 and 16, Good and Super Rods can catch only Poliwhirls and Basculins, each giving you 2 Speed EV points. A chain of 42 with a Power Anklet on and a pokémon will max out its 252 Speed EVs. The same applies to a Super Rod on Routes 3 and 22 with Attack EVs, Gyarados, Seakings, Crawdaunts and Sharpedo all give you 2 Attack EV points while a Good Rod can net you Goldeen, Corphish and Carvanha all giving you 1 Attack EV. If you are looking for Special Attack EVs try a Good Rod in Cyllage City or Ambrette Town in pokémon X. Corphish and Horsea each give you 1 Special Attack EV.
Keep a Sticky Hold or Suction Cup pokémon in the first spot and bring a couple pokémon that can handle Water types, maybe some Leppa berries and try and faint them all. Feel free to switch Power Items any time you want, it won’t affect the chain. Just don’t leave the area until your chain ends. Even if I don’t get a shiny, at least I was able to accomplish some EV Training.

Friend Safari Training
There are only three pokémon in each safari and they share a type to enable you to exploit a common weakness. The biggest advantage of EV training in Friend Safari is the greater likelihood of finding shiny pokémon there.

For more specifics on what EVs are there and where to find them check out this thread.