You will need two people.
1.) Find a repeatable collection quest, i.e. kill spiders for 10 spider nutsacks and turn them in to captain blah.
The higher the level of the quest in relation to you the better, reds are best so long as you have a high level to kill the mobs for you to allow you to loot the quest items.
2.) Go to the turnin guy.
3.) Have your friend open a trade window with you.
4.) You then proceed through the quest turnin dialogue and accept the repeat of the quest.
5.) Next you hit accept on the trade window.
6.) Now your friend does the same.
You've got the xp and credit for the quest, but you still have your 10 spider nutsacks to turn in again.
Repeatable quests give good xp and it usually takes 4 or 5 turnins a bubble, but after some practice you can get it rolling pretty fast especially if you use the trade slash commands and hotkey them.
/trade (to open a trade)
/tradeyes (to accept a trade invite)
/tradeok (to accept trade)
I hope you enjoy this as much as I do.