You should only use Mana Shield in specific situations when you have a Mana Gem up.
At that level your main deal is going to be Polymorph, heal, reinitiate the fight. The real strength of a Mage is the kiting ability but there are almost as many things to negate kiting as there are Fearing. Your best friends are going to be Blink and Iceblock.
In Arena teams and WPVP, where Iceblock is not so commonly used or are refreshed, these tools are beyond lifesafers. They allow you to rework the field to your advantage. Hunter Reds, Pally Bubbles, Rogue Cloaks?? Just IBlock and wait it out. If you set up a Macro correctly you can make it on 1 click it Blocks you, on second click it unblocks you. Use this to break when it seems smart (sometimes immediately after, sometimes as long as you can wait) and then Frost Nova -> Blink away.
Fact is, though, where its relatively easy to get Damage increases in melee stats through STR and AGI, its next to impossible to get anything "good" in the spell power department until TBC. Its simply not balanced. It's something you'll have to deal with until around 60, when Deep Freeze or Arcane Barrage become options. And Ret paladins, Warriors, Rogues, and Hunters ability to catch someone who is fleeing is especially nasty.
TBH you're just going to have to put up with it until then. Later on, tools like Invisible and Mirror Image coupled with high Spell Power will help you versus these baddies... and there will always be classes to give you problems like Prot Warriors and Unholy Deathknights.
Edited, Jan 19th 2009 12:16am by tzsjynx