Originally written by a friend of mine who was unable to post this herself.

The Basics

When a pokemon is freshly caught or newly hatched, it has 0 EVs. It can max out at 510 EVs.
Every 4 EVs give one point to a certain stat. Each stat can only have 255 EVs total.
When you give a stat 255 EVs, however, you are wasting 3 EVs that don't make a point. The recommended maximum is 252.
So say you have two stats. Attack and Speed.

Attack: 252 EVs
+Speed: 252 EVs
504 EVs total (6 remaining)
You still have 6 EVs. So naturally, you'd put that one extra point into some other helpful stat, like HP.

Vitamins

HP Up - Increases HP by 10 EVs
Protein - Increases Attack by 10 EVs
Iron - Increases Defense by 10 EVs
Carbos - Increases Speed by 10 EVs
Calcium - Increases Special Attack by 10 EVs
Zinc - Increases Special Defense by 10 EVs

You can only use these on the first 100 EVs. Only 10 can be used for each stat.

[NOTE: Vitamins are not doubled by Pokerus]

In the 5th generation, you have a new item called wings. They are as follows:

Health Wing: 1 HP EV
Muscle Wing: 1 Attack EV
Resist Wing: 1 Defense EV
Genius Wing: 1 Sp. Attack EV
Clever Wing: 1 Sp. Defense EV
Swift Wing: 1 Speed EV
Pretty Wing: IT DOES NOTHING. So just sell them.

These are unlimited, unlike the 10 vitamin per stat limit. Use as many as you want/can.
You find these in the shadows of birds on Marvelous Bridge and the Driftveil Drawbridge.

[NOTE: Also not affected by Pokerus]

Pokerus

To get an affected Pokemon with Pokerus... I dunno. Ask your friends. It's too hard to get naturally. Very small chance.
You can spread it when your pokemon is infected. Put the infected in first slot of the party, put to-be-infected into the second slot.
Jump in and out of battle a couple of times. The second Pokemon should get Pokerus.

[NOTE: at midnight, all pokerused pokemon in your party will become immune. They will still get the benefits, though.]

What it does:
Pokerus is a condition when your Pokemon gains double the amount of EVs. For example, killing a Lillipup with pokerus will give you 2 Attack EVs instead of 1.
Not a big difference?
200 Lillipups vs 100 Lillipups.

Power Items

Macho Brace: Doubles any EVs gained in battle. [With Pokerus, it means EV Gained x 4]
Power Weight: Adds 4 HP EVs after every battle. [Pokerus = 8]
Power Bracer: Adds 4 Attack EVs after every battle. [Pokerus = 8]
Power Belt: Adds 4 Defense EVs after every battle. [Pokerus = 8]
Power Lens: Adds 4 Special Attack EVs after every battle. [Pokerus = 8]
Power Band: Adds 4 Special Defense EVs after every battle. [Pokerus = 8]
Power Anklet: Adds 4 Speed EVs after every battle. [Pokerus = 8]

In Generation 5, these can only be bought via Subway BP.
But you get Macho Brace for free. I don't remember where.

Power items cut Speed when you're wearing them. Do not worry - it is not permanent.

Anyways, killing 1 Lillipup + Power Bracer + Pokerus =

[1 EV + 4 EV] x 2

= 10 Attack EVs per Lillipup. ^w^

Exp. Share

Yes, you can pass EVs around like this. Pokerus does double your gain.
BUT - the power item bonuses aren't given. Power item bonus can only work if the pokemon HOLDING the item is sent into battle. So yargh.

EV Erasing Berries

This is for when you are attached to a pokemon, but it's not EV trained to its full potential.
You simply wipe the unwanted EVs with these berries (can only get from Dream World) and re-train them.

Pomeg: -10 HP EVs
Kelpsy:-10 Attack EVs
Qualot:-10 Defense EVs
Hondew:-10 Sp. Attack EVs
Grepa:-10 Sp. Defense EVs
Tamato:-10 Speed EVs
Some Pokemon just do not need some of these. For example:
Would you give Shedinja 252 HP EVs?
To fully wipe a stat, 26 berries are needed (252 - 260 = 0. LOL.)
Editor note: In Gen IV and III, if your EVs were above 100, the first EV berry would automatically set you to 100, no matter what. That means only 11 were needed maximum.
Oh, your pokemon will also be more friendlier with every berry.

BW1 Locations

HP = Stunfisk (2 EVs) on the waters of Iccirus. These little douchebags use Endure a lot, so counting PP usage may not be the most reliable way to tally EVs for them.
But if you're a lucky bastard and didn't catch Victini yet, you can kill it for 3 HP EVs every time. It respawns infinitely. So if you haven't caught it, don't try to catch it.

Attack = Lillipups and Patrats (1 EV each). Route 1. It's a massacre.
Editor's note: I personally prefer Dragonspiral Tower for higher levels. When you use Surf and go to the right, the dark patch of grass gives only Pokemon that raise Attack EVs, except in Winter

Defense = This one's a toughie. There's no decent Defense EV places in all of Unova. I usually just equip the Power Belt (defense) while I EV in a different stat. So complicated.
Editor's note: The best place is Pinwheel Forest, where you can KO Sewaddles and Venipedes for 1 EV each. Petilil/Cottonee likes to mess it up, though

Special Attack = Litwicks and Elgyem (1 EV each). In Celestial Tower. There is a person there to heal your pokemon. No need to spam potions or run back to the Poke Center.

Special Defense = You COULD kill Frillish. Frillish everywhere for 1 EV each. But I prefer running to the Volcarona room and killing Claydols for 2 EVs each. You'll just need a higher leveled pokemon to do the job. Level 60-70 for OHKOs.
Editor's note: Chinchou (1 EV) and Lanturn (2 EVs) also appear when fishing ripples where you find Frillish.

Speed = Basculin give 2 Speed EVs each. Just surf on that water in Route 3.

BW2 Locations

Added by Shaquiel (#15)
HP

Grimer appears between levels 5 and 15 in the Castelia Sewers in Spring and Summer only, and yields 1 HP EV. In the waters of Icirrus and Route 8, you can find Stunfisk and Palpitoad for 2 EVs each. Seismitoad (3 EVs) seldom appears in ripples in Icirrus.

Attack

Golurk and Banette, holding 2 EVs, are on Victory Road's first and third floors. Dragonspiral Tower's first floor also holds Druddigon and Mienshao, also giving 2 EVs. Druddigon and Zweilous appear in N's Castle, with Zweilous giving up 2 EVs as well. Excadrill hides in dust clouds with 2 EVs, but Onix is in these same dust clouds and will give you Defense EVs. Nowhere else has anywhere near the same encounter rate with Attack EV Pokemon.

Defense

Unfortunately, like BW1, no area exists with 100% encounters with Defense EV Pokemon. Sandshrew and Yamask each have 1 EV and appear in Relic Castle, for lower levels. The other three appear in Chargestone Cave where Boldore also dwells with 1 Defense EV and 1 Attack EV.

Sp. Attack

Like BW1, these two appear in Celestial Tower with 1 EV each. You must stay on the first floor to avoid Golbats.

Sp. Defense

Frillish (1 EV) appears in a few places while Surfing with a 100% encounter rate, such as Route 4, Virbank City, and Route 17. Jellicent (2 EVs) also appears in ripples on Route 17. Baltoy is in Relic Castle, in the same room as Volcarona, where it gives 1 EV.

Speed

Route 19's surfing area has Basculin only, much like Route 3 in BW1. It gives 2 EVs.