this, this and more of this.
Most importantly because of your class, practicing Arena and learning how to react to what other people is doing is key. It doesn't matter so much for other classes, but as a Mage you are the main source of damage.. so getting kicked on your frost tree during all your cooldowns is pretty shit. So is getting kicked on frost and then getting oneshot because you're over extending and can't block.
It'll all come with experience and watching better players.