If you want to have more HP:
Get a con boosting item. This will help with your HP
Bear's Endurance might help you, but like with barbarian rage, the HP gained will go away after the spell runs out - so be sure that you are healed sooner than 12 minutes after you cast that spell (or extend it).
The spell "false life" (sor/wiz 2) is your friend. It will give you 1d10+10 temporary hit points, and this for 12 hours. (I hope you didn't ban Necromancy!) You might want to empower and/or extend that one. (or maximize).
The spell "vampiric touch" (sor/wiz 3) works even better, but you need someone to touch (so you either have to wait for combat and actually touch a living enemy, or you have to make a deal with the cleric that you can use it on him and he heals himself). On your level, we're talking about 6d6 temporary hit points, again for 12 hours. As with false life, use empower, extend and maximize depending on your preferences.
If you want to decrease the damage you are dealt:
You might want to use stone skin, but only if it really becomes necessary - after all, it will cost you 250 gp! Protection from Arrows might be appropriate if you encounter archers on a regular basis, and only seldomly are directly involved in melee.
Mirror Image will also help you suck up a couple of attacks. Displacement and Improved Invisibility will make it less likely that you're hit.
Get some sort of energy resistance: maybe a combination of "protection from energy" (for the type of energy you encounter the most) and "resist energy" or magic items for one or more of the other energy types.
Increase your AC via cat's grace, mage armor (unless you already wear bracers of armor +4 or better) and shield.