Before I begin, I want to say that this section has nothing to do with Breeding whatsoever, but it is a good thing to know when it comes to training a pokemon.
You may have heard about Effort Values, or EVs, somewhere before. EVs are another factor that determines how strong your pokemon will be in the end. Unlike IVs, which are randomly determined strictly from birth, you actually have full control over what EVs you're pokemon will have.
All pokemon are able to get 510 total EVs. For every 4 EVs in one stat, that stat will get an extra point. Because 4 is not evenly distributed into 510, 2 EVs will go unused, so only 508 of the 510 EVs really count. Also, it should be noted that each stat can only get a maximum of 255 EVs into(again, not evenly divisable by 4, so really only 252 count).
How does one get EVS you ask? If you shut up and have some patience, I'll tell you. You get EVs through the use of Vitamins, Wings, the Power items, and battling. All pokemon give out a certain amount of EVs to one or two stats, from 1 to 3 EVs in a single stat. A good way to find out what pokemon give what EVs is here;
http://www.serebii.net/pokedex-bw/
Vitamins will give +10 EVs to their respective stats with each Vitamin, and you can use up to 10 of a single Vitamin for a single stat, meaning at lv 1, you can have +100 EVs already on one of your pokemon's stats. You can buy Vitamins for $9800 at the Pokemalls in any region.
Wings are similar to Vitamins. There are 2 differences, however. A Wing will give just +1 EV to its stat. You can also use a limitless number of Wings on a pokemon, however. So if you had 510 Wings, you could have a pokemon fully-EV trained at lv 1, without using Vitamins. Wings are also the only way to EV train a pokemon at lv 100. Wings can only be found on the bridges of Unova(not including Skyarrow or Tubeline).
The Power items that I mentioned in the IV post, will give you +4 EV points in its respective stat at the end of a battle, along with the EVs given by defeating the pokemon itself. For example, if you send in your pokemon holding a Power Belt to fight a Durant, and you win, you will get +2 Defense EVs for killing the Durant, and you will also get +4 Defense EVs for holding the Power Belt. Same scenario, but you're fighting a Victrebell, you would get +3 Attack EVs for killing the Victrebell, and +4 Defense EVs for holding the Power Belt. The Macho Brace is similar to the Power items. It doubles the EVs the holder receives from a battle. Though inferior, it is found free of charge on your journey, so you can make use of it if you haven't gotten the BP for the Power items. You'll find it in Nimbasa City in Generation V, and in Generation IV a trainer will give it to you if you show him a set of Pokemon he is looking for.
Then there is the magical "Pokerus" virus. Nearly impossible to obtain, but if you do get it, cherish it. A pokemon with the Pokerus virus will get DOUBLE EVs through battle. Using the above example, you kill a Durant while holding the Power Belt, and you have Pokerus; you will get +4 Defense EVs for killing the Durant, and +8 for holding the Power Belt, a total of +12 Defense EVs from a single battle.
I'll go into the rules of Pokerus more. You get Pokerus from doing wild battles...and you have a very small chance to get it(just less than the odds of finding a wild shiny pokemon, if that gives you an idea). That's the hard part. Once you get it though, it's easy to keep. Pokerus is cured after midnight comes around, and a little mark will appear on your pokemon's summary saying it used to have Pokerus. This pokemon will still recieve the benefits of Pokerus, but will be unable to spread it to other pokemon. However, a pokemon in the PC will keep Pokerus in its full, so always make sure you have a spare in you PC. To spread Pokerus, you just battle, and there is a 50-50 shot of the Pokerus spreading to other pokemon in your party. If the Pokerus spreads, it will spread to the pokemon before and after itself in the line up.
The Exp. Share is the last tool. Most know it for splitting experience between 2 Pokemon, but most are not aware that it also gives the holder the base EV gain from that Pokemon. For example, if you go into battle with a Machoke against a Basculin, and you have a Bagon in your party holding the Exp. Share. The Machoke will get the 2 Speed EVs for killing it, and the Bagon will also get 2 Speed EVs, 4 if the Bagon has Pokerus. You don't get as many EVs as if the Bagon had held a Power item and been in the fight himself, but you do save the time and trouble from switching out.
Some FAQs now;
Q: How does one keep track of their EVs, Oh-Mighty Masterbreeder?
A: Firstly, thank you, that is high-praise indeed. The only way to keep track of your EVs is with a pencil and paper, I'm afraid. There is a person ingame, however, that will tell you if you've maxed out your EVs or not. You can find her in Opelucid City(I can't recal where in Sinnoh, however). She'll say something along the lines of "Your pokemon put in a great effort". But she'll only tell you if you maxed out or not.
Q: What generation does this apply to?
A: I'm not answering unless I hear some praise.
Q: What generation does EV training apply to, oh Lord of Breed?
A: That's better. It applies going back to the original Gold and Silver games, if not to the very beginning. Wings were not introduced until Generation V, however, and Power items are an addition from Generation IV.
Q: Are there any ways to lower my Pokemon's EVs once he's gotten them, Prince of the Egg?
A: Oh stop it, you're making me blush. The only way to lower EVs is through the use of certain berries. They are the Pomeg, Kelpsy, Qualot, Hondew, Grepa, and Tamato Berries. Each removes 10 EVs in their respective stat. You can use these Berries to fix any mistakes you may have made while EV Training. In Generation V, these are only found in the Dream World. They are found in the wild in Generations III and IV.
Q: What are some good locations for EV breeding, oh magnificant King of the Egg?
A: Come up with your own praise, geez. It all depends on what you're trying to raise. I'll list here;
HP: Stunfisk are garenteed to be found in the dark spots in the water of Iccurus City, he is worth +2 HP EVs. If you have the Victini event, you can kill Victini for +3 HP EVs, exit the room, and go back in to do it all over again.
Attack: Patrat and Lillipup are also garenteed to be found on Route 1, giving +1 Attack EV each, and all the pokemon outside Dragonspiral Tower, in the thick grass, give +2 Attack EVs(except for Mienfoo, who gives +1 Attack).
NOTE: In Winter, Vannilish can appear, who give Sp. Attack, not Attack.
Defense: Durant are found fairly often in Victory Road, and they give +2 Defense EVs. Roggenrola that are found fairly often in the Wellspring Cave give +1 Defense EV each. There is no place garenteed to let you encounter a Defense EV pokemon.
Sp. Attack: Liwick and Elgyem are garenteed to be found in the Celestial Tower, and each give out +1 Sp. Attack EV.
Sp. Defense: Frillish are garenteed to be found in the waters of Driftveil City, and give out +1 Sp. Defense EV.
Speed: Basculin are garenteed to be found in a lot of water, but I'm gonna list just Route 3. They give +2 Speed EVs.
So any other questions or comments?
Brock Out.