I normally like equipping an Exp.Share to whatever pokemon I want to level, then run through the Elite 4 with that pokemon in front. It goes out first, and then you switch to whatever you're gonna kill the pokemon with. Then, when the person sends out their next pokemon and it asks you to switch, switch back to the Exp.Share guy, and repeat.
Example:
The foes Drapion was knocked out!
XXX is about to send in Beautifly!
Will you switch pokemon? (Yes)
Come back, Celebi!
Go, Dratini!
XXX sent out Beautifly!
(First move)
Return, Dratini!
Go, Celebi!
With this setup, I believe the Exp.Sharer gets 75% of the experience or something like that.