"Fragile Wizards"
For most armies, calling their Wizards fragile is fair and honest - they're inevitably not as tough or good at fighting as their warrior counterparts. Chaos Sorcerers are, thankfully, better than most and they have access to some options to keep them alive and kicking against whatever comes their way.
My favourite option is the Warrior Familiar - who inflicts an automatic S5 hit on a single enemy model before blows are struck. Perfect for getting the edge over a powerful opponent, especially if you're likely to end up in a challenge against him. In terms of defence, consider decking your Chaos Sorcerer out with a suit of Magic Armour, such as the Bronze Armour of Zhrakk (oh, and despite the scurrilous advice I've given in the past, you can't equip your Sorcerers with magic shields... and apparently no amount of wishing, arguing or whining on my part will make it true).