EV Guide

EV Training Guide
EV is the short name for Effort Values.By EV training your pokemon,your pokemon gets stronger and ready for battle.A pokemon can only hold 510 EVs total.One pokemon's Stat[Stat is the HP or Atk or DEf or Speed or SP DEF or SP ATK]of a pokemon can only hold 255 EVs.I would say 252 EVs are better to give your pokemon.
Power Items
You can speed the EV training process up using vitamins.
HP Up -10 HP EVs
Protein -10 Attack EVs
Iron -10 Defense EVs
Calcium-10 Special Attack EVs
Zinc -10 Special Defense EVs
Carbos -10 Speed EVs

And you can speed up the EV training process by killing pokemon while holding these items;
Macho Brace
The Macho Brace is an item that gains you double the EVs you get when you defeat a normal pokemon.
eg. You are holding the macho brace and you defeated a Starly which would normaly give you 1 EV in speed,now you get 2 EVs instead of 1 EV.But the only problem is that it cuts your speed stat while battling.

Best Power Items
Power Weight - HP
Power Bracer - Attack
Power Belt - Defense
Power Lens - Special Attack
Power Band - Special Defense
Power Anklet - Speed
The power items are made for all stats.You can double the EVs when you hold this item while battling wild pokemon and defeating them.
Eg.You are battling a wild Starly[1EV point in speed] and you are holding the Power Anklet[which ups speed EVs] on your pokemon.You defeated the starly and instead of gaining 1 EV point for speed,you would gain 2 EV points.

Exp Share
The EXP Share is another useful item for EV training as EVs are gained each time experience is gained. If a Pokémon in your party is holding the EXP Share item, it will gain the same EVs as if it was the active Pokémon in battle.Note that the pokemon holding the Exp share does not gain the extra boost if your other pokemon is holding a Power Item.

EV reducing Berries
There are berries that reduce your pokemon's EVs so you can EV train them over.
Here are a list of does berries;
#21 Pomeg Berry - HP
#22 Kelpsy Berry - Attack
#23 Qualot Berry - Defense
#24 Hondew Berry - Special Attack
#25 Grepa Berry - Special Defense
#26 Tamato Berry - Speed
Using 11 of the berries on one stat would reduce all the EVs[255EVs] in a pokemon's stat if you are unsure.

Hot Spots to EV train
HP
Route 209.
Trainer is Cowgirl Shelly. Location; Cycle south out of Solaceon Town to find Cowgirl Shelley at the top of the bicycle ramp.
Has 5 bidoof[s].
Bidoof emit 1 HP EV each.

ATK
You can fight Kricketune and Bibarel which worths 2 EV points each at route 212.
Or
Use the Super Rod at Route 229.
Only Gyarados appears here.
Gyarados emit 2 attack EVs each.

SP ATK
Eterna Forest, Old Chateau.
Gastly, Haunter and Gengar. Gengar has a 4% chance of appearing in the rightmost room on the second floor if either FireRed, LeafGreen, Ruby, Sapphire or Emerald GBA games are inserted into your DS.
Gastly emits 1 special attack EV each, Haunter emits 2 special attack EVs each and Gengar emits 3 special attack EVs each.

Note,to gain the two stats at the same time,battle an Unown to gain 1 ATK EV and 1 SP ATK EV.

DEF
Route 214.
Ruin Maniac Cave. Head south from Veilstone City.
Hippopotas and Geodude.
Hippopotas and Geodude both emit 1 defense EV each.
Or
At Iron Island you can find Graveler worth 2 DEF EVs each and a Steelix worth 2 DEF EVs each.

SP DEF
Route 220 surfing on water.
Tentacool and Tentacruel. Run from any other Pokémon that may appear here.
Tentacool emits 1 special defense EV and Tentacruel emits 2 special defense EVs.

Speed
Route 205.
Fisherman Andrew. Fisherman Andrew is located on the middle panel of route 205’s bridge.
Has 6 Magikarps.
Magikarp emit 1 speed EV each.

Pokérus
Pokérus is a Pokémon virus that will double the amount of EVs you earn in battle after all item increases. For example, if Lapras with Pokérus battles and defeats Floatzel it will gain 4 speed EVs, instead of the usual 2 speed EVs. If Lapras with Pokérus was holding the Power Anklet it would gain 2 speed EVs from Floatzel plus 4 speed EVs from the Power Anklet, this would then be doubled by Pokérus to give an end result of 12 speed EVs. Finally, if Lapras was holding the Power Bracer it would gain 2 speed EVs from Floatzel plus 4 attack EVs, these would then both be doubled by Pokérus to give 4 Speed EVs and 8 attack EVs. Pokérus will not multiply the effect of HP Up, Protein, Iron, Calcium, Zinc and Carbos.

EV Training Tips
Here's some advice from a person who EV trains
EV training can be completed at any level but it is best to complete EV training on your Pokémon at as low a level as possible to make leveling up your Pokémon an easier process. If you complete EV training at low levels you no longer need to worry about recording EVs and it becomes faster to reach LV100.

Different people will prefer different methods but I would recommend to always use the Power Items and start EV training from LV1. Attach the Power Item to the Pokémon you would like to EV train and set it to slot 1 in your party. Have a high leveled Pokémon in your party too. When you battle, your low level Pokémon holding the Power Item will be sent out. Immediately shift on your first turn to send out your high leveled Pokémon to end the battle and gain the EVs. This situation would work similarly to the EXP share method; the Pokémon holding the Power Item will gain the EVs from the Pokémon defeated and from the Power Item (even though it is shifting out of battle, it will still gain EXP, therefore it will still gain EVs,) and the Pokémon that switches in will only gain EVs from the Pokémon defeated.

Also note that if you accidently encounter a wild Pokémon that you do not wish to gain EVs from, simply run from the battle and no EVs will be gained.

Record the EVs that you have obtained. When you have completed EV training visit the lady in the Sunyshore Market. If your Pokémon has reached 510 EVs she will reward the Pokémon an Effort Ribbon. This is a good way to check that you have recorded your EVs correctly.

My advice
[1]A pokemon worth 2 EVs was killed by a pokemon that is holding a Power Item and has Pokérus would have to kill that pokemon 21 times to gain 252 EVs in the stat.eg.2[2+4] = 4+8 =12 EVs
[2]A pokemon which is worth 1 EV and all the above would have to kill the pokemon 42 times to gain 252 EVs in that stat.eg. 2[1+2]= 2+4=6.
[3]A pokemon which is worth 3 EVs and all the above would have to kill the pokemon 14 times to gain 252 EVs in that stat.
eg. 2[3+6] = 6+12=18.
Eg. of working out stats...
[1] Pokérus>>> 2[2+4] 2 is normal EVs gained.
4 is Power Item boost.
Nature Update
You would need to have the right nature for the right pokemon to make it strong.
Natures
Natures are the mood of pokemon.They affect how good a pokemon's stat is.Here is a list of natures on what affect what's stat.
ATK
Lonely [+ATK -DEF]
Brave [+ATK -Speed]
Adamant [+ATK -SP ATK]
Naughty [+ATK -SPDEF]

DEF
Bold [+DEF -ATK]
Relaxed [+DEF -Speed]
Impish [+DEF -SP ATK]
Lax [+DEF -SP DEF]

SP ATK
Modest [+SP ATK -ATK]
Mild [+SP ATK -DEF]
Quiet [+SP ATK -Speed]
Rask [+SP ATK -SP DEF]

SP DEF
Calm [+SP DEF -ATK]
Gentle [+SP DEF -DEF]
Sassy [+SP DEF -Speed]
Careful [+SP DEF -SP ATK]

Speed
Timid [+Speed -ATK]
Hasty [+Speed -DEF]
Jolly [+Speed -SP ATK]
Naive [+Speed -SP DEF]

Natures that does not affect any stat
Bashful
Docile
Hardy
Serious
Quirky