My tricks for levelling: First things first, I use Rare Candy. Only for a few levels, at least enough to OHKO most EV opponents. If it's just HP I'm after, then L12 will do, but other stuff needs higher levels. I usually use Vitamins to speed up the EV training, since I don't have loads of time to play the games.
My choice of level training ground is not the Elite Four. Nor is it the Sky Pillar. It's actually Route 117. Why? Well, three reasons really?
One - it's close to two Pokemon centres for quick healing. Verdanturf is to the left, and Mauville to the right.
Two - there are 5 Trainers Eyes trainers on the route (these are the only things on the route I battle, the wild Pokemon are too low). At their max level (between L31-L37) the trainers' Pokemon give out as much Exp as the Sky Pillar, with 20 lower levels to deal with. It also gives me loads of cash (which is why I have over 600,000 cash, and I've only ever battled the Elite Four ONCE).
Three - it's quick and easy. Just exit and enter the route a few times, and a trainer will rebattle you. Two of those trainers has 6 Pokemon each, so that gives around 6000 Exp per battle (that's around 1/6 of a level at L90, not a bad chunk).
Oh, and of course the Daycare Centre is also on this route, so it's become my personal base of operations (being the breeding fan/expert that I am).