As a mage you have the following defensive tools : IB, Frost Ward, Fire Ward, Demoniac Healthstone, Healing Potion (or Engineers Injectors), Mana Shield, and Combo of Mana Shield and Mana Sapphire.
Ice Block: 5 differents cases of utilization
Absorb fatal damage (a one shot or multiple times during a short period - e.g Blood Queen Bloodbolt Whirl)
Survive for a short time if a healers are too busy to keep you alive.
Dispelling a debuff you cannot dispel yourself (Remove Curse). Use it this way if the debuff will kill you.
To give back the boss' aggro to the tank, buying them time to build threat again (it doesn't cancel aggro, it only prevents you to have the boss threat during the time you are in the block.)
Maintain a usefull position during attack on which you will have usually to moove (e.g. Valanar Shock Wave). This way you will not have to blink to coming back in the totems and other auras areas (or even just at a proper range to hit the targets) [Special thanks to Khadgar on this point]
Ice Block has a 5 minutes cooldown so use it strategically.
Wards (Frost and Fire): Abuse it (but remember that it takes a GCD, so only if your healers have issues). It's one of your two best defensive abilities to give some breathing space to your healers. And do not be mistaken, every heal they don't have to do it's a bit of mana saved which will be useful for your raid.
Here is a list of spots you can use them in ICC :
Lord Marrowgar: Before Coldflames (frost)
Lady Deathwhisper: Frostbolt (frost)
Gunship: Missiles damaging AoE (fire)
Taldaram: Conjured Flames (fire)
Valanar: Kinectic Bombs (fire)
Valithria Dreamwalker: Mages' Frostbolt (frost)
Sindragosa: during the whole fight (frost), and especially before the frost tombs (frost)
Lich King: If you are target by an Ice Sphere (frost). If you will be hit by Remorseless Winter.
Mana Shield: Try to never use it, the only one case it will be useful is if you see you will take a huge amount of damage that healers will not handle. For example: You have accumulated too many stacks of instability on Sindragosa, you already have used your IB, so at this moment use you Mana Shield to survive, you will absorb approximately 4k damage and with a properly-used frost ward you will give the healers enough time to heal you. Moreover in the present case (Sindragosa), losing mana is not really a problem because you have a lot of idle time to evocate.
Healthstone (Warlock): Restores health in a critical moment.
Health Potion: Restores health in a critical moment.
Combo ManaGem(or Mana Potion) + Mana Shield: this way you will absorb almost freely 4k damage (in reality it cost you the mana pool you will not have back later when you will need it).
WARNING: potions (Mana, Health, Speed, Wild Magic) and Engineering Injectors, all share the same cooldown, and can only be used one time during combat.
You have many useful spells, do not hesitate to abuse them : Blink, Spellsteal, Remove Curse. You have no healing abilities so do not forget to bind your health potions.