I have had over 50 pokemon at level 100 in all my games - raised naturally, that is. In my . That's a rather conservative estimate, actually. Still, I simply enjoyed raising them and seeing their final, mind-blowingly awesome stats. XD
I stopped doing it in Diamond, actually, because I realized that it was moot when there was an automatic levelling function for wifi.
As for how, beating the Elite is a good method, but only once they are high enough level to handle them all easily. The Elite usually have fairly tough pokemon with decent movesets, so it may be more worth your time to find pokemon with lower level pokemon that you can beat faster.
The best places for raising levels fast were, sadly, removed in Platinum. There were two breeders just north of the breeding town that had Happiny at around level 17. They netted a huge amount of EXP for low-level pokemon and were fairly easy to beat. Another good person to train against was a Cool Trainer on the route with the constant sandstorms on the battle island. (S)he had an Ambipom, Rapidash, and Blissey. Blissey gave nearly 5000 EXP with Lucky Egg, and was not hard for physical attackers to defeat.
In Platinum, there is only one breeder with a Happiny, and the Blissey trainer has a whole-new team. I have yet to find as good a spot for raising yet.