The most effective way to gain EVs in Gen VI (unless you can hack in Wings, which is a viable alternative in Gen V and on) is to use Super Training.
Super training can be convenient for grabbing small numbers of EVs to finish out a spread or hit an unusual number like 12, but for racking up EVs quickly, EV training with hordes is the best bet.
Say you want to max a certain stat's EVs. First, check Nerd's guide
here for an appropriate horde location. Equip the Pokemon you want to train with the needed boosting item (Power Anklet, Weight, etc.), which adds 4 EVs in that stat for *each* enemy Pokemon you knock out. Make sure you have a lead Pokemon with a powerful spread attack, and a Pokemon with Sweet Scent. Turn on the Exp Share, and go to the appropriate zone. Use Sweet Scent on terrain where wild Pokemon appear to trigger a horde battle. OHKO the horde with your spread move. Boom, 25 EVs gained. (1 EV + 4 from Power Item per Pokemon) * 5 Pokemon per horde = 25 EVs. Without vitamins, this means that 10 horde battles will get a stat to 250 EVs, requiring just 2 more evs to max the stat, which can be gained with another horde battle, super training level 1, etc. If you boost that stat 10 times with the appropriate vitamin first, just 6 horde battles are needed to get to 250. Better yet, thanks to the Exp Share, you can EV train multiple Pokemon in a stat simultaneously this way, making it an even more efficient training method.
Also, note that unlike in previous generations, in 6th gen Pokemon, EVs cap at 252, not 255. This means it's impossible to waste 3 EVs by over EVing when you max a stat, so you don't need to stress about that.