How do I EV train my pokemon?

So I'm playing Pokemon Black, and I'm wondering how do I EV train my pokemon? I know that I gain effort values from battling pokemon, and that a maximum of 510 EV's can be gained, but what 510 EVs count, the first 510 I ever receive, or are they continuously replacing each other and it's the last 510 EV's I received in battle? My pokemon are approaching level 80 now so I want to know whether I'm too late, or whether I can still beef the up to my favour.