Not really an exploit but its pretty darn easy...
Apparently if you sit in town with your pet, your pet levels. As for the speed of leveling, I'm not so sure however, if you feed him/her/it/the pet in the morning and when you come home and maintain its loyalty...its an easy occupation and way to gain access to strong pets to help you on your quests or levels or simply for selling and making money. >:)
Note:I'm not sure if this has been mentioned already.
your pets health wont go down but his LOYALTY will and he will RUN AWAY unless you have a macro to feed him. And he gets 1xp per 3 minutes so it will take you 10 years to level him this way and more food then its really worth.