Not sure if this has been posted before, sorry if it has
Thought this was common knowledge but there are always new people =)
When you are leveling with a mate for some reason or another one of you always lags behind. Using this method you can help them catch up a bit.
Blizzard changed the way you gain xp from a kill, it is now based on the damage you deal, however if the person that kills your tagged mob is within the level range of said mob, then you get full xp.
So say you tag x mob, then your friend runs in and kills it, you will get full xp.
The mob must be at least green to your partner.
Oh and don't be in same party >.<