Okay I think this is pretty commonly known among mages, but I'm sure there is many that dont. This is mainly for Arcane Mages but, can be useful to others.
Alright well many of you may or may not have noticed, that when your enemy goes behind a wall or something, during while you were channeling arcane missiles, the missiles kept going until the 5 seconds was up. This comes in handy a lot for me.
What you need to do:
Preferably have some points in imp arcane missile, and other arcane traits for this.
Cast arcane missiles at a running enemy if hes about to turn a corner
Let the arcane missiles keep going as he goes out of LOS, which of course shouldn't happen when they're out of your LOS.
As an arcane mage I find this useful in PVP because many times they run, and end up dying when they cut a corner because my arcane missiles doesn't stop for LOS.
Mages, ENJOY.
Others, beware.