PVP help

Hey guys, my mage is 68 or so and i've tried all three trees for pvp. They're all ok. The biggest problem i'm having is that i get focused all the time and have a hard time trying to stand still casting frostbolt/arcane blast/fireball in bgs. Any suggestions for mass type of pvp where you have like 4 on 4 and so where you get constantly focused?