I recently found this out myself after much trial and error. Here is what you will need:
1.) An alt between 10 and 40 for maximum efficiency
2.) A friend who is either a hunter, or a felguard specced warlock
3.) A character you can park inside of the instance and keep at a distance from yourself(preferabley a rogue for stealth)
4.) A macro for /invite "player"(trust me, you'll want this)
I'll use Scarlet Monastery as an example to explain this. What you do is invite your friend, and the character you are going to park to a group. Once grouped, bring in the char that is going to be parked and park it far, but not too far, away from wherever you are at the time. Basically you want it far enough that it wont leach xp, but close enough that the mechanics of being grouped treat it as if he is near you.
Once you have a char parked far enough away from you, have your warlock or hunter buddy drop group. Once hes out, tag a mob, and have your hunter or warlock kill it with ONLY his pet. Generally only takes a few seconds per mob. You will get full xp and all loot. And because they are instance mobs you receive loads of xp very fast. After the mob is dead, or your friend is close to being ported, reinvite him. And repeat for the whole instance, I was getting my friend a level every 45 mins in SM Cathedral using this method. Very efficient if you get it down well.
The reason this works is because when grouped and near a fellow party member, XP is not calculated by your damage done to the target, unless someone outside the party helps kill it. But since a pet is an npc, it doesnt effect the amount of xp you receive.
If anyone has any questions feel free to ask!