Playing a Caster can be a fun though rather challenging experience. Oftentimes, a caster is essential for completing a tough quest as there are a decent amount of monsters who can only be affected properly through magic. Besides, there are plenty of specific terrain features in some quests where a good caster can drop a Firewall, Wall of Fog or Cloudkill to decimate a group of enemies while everyone watches. The single greatest enemy a Caster faces is damage itself; they usually have a poor armor class, poor attack rating and low hit points. In fact, a single well placed critical can even take down a higher level caster as well.
This is where a good playing strategy is essential on keeping him alive during those tough fights. I have taken some time reflecting on how I play Casters and seen other people play them and assembled a few tips on keeping your Caster alive in a dungeon.

1. Always keep a tower shield on your hotbar. Despite how silly this might initially seem considering the tower shield's high arcane spell failure chance and attack roll penalties, it is one of the most effective defense measures for a caster. The idea here is to get your hands on a magic tower shield. Give it a spot on your hotbar and make sure you have a staff or missle weapon spot as well so you can switch between the two as needed. Simple plan is that when you MM or else blast a bad guy, it instantly aggros on you so that's the time to go into tower shield and blocking mode. You would get a shield AC bonus of anywhere from +4 to +9 and then an additional +2 AC from blocking as well as decent damage reduction. While the bad guy beats on your tower shield as you are blocking, the tanks come in and pound the bad guy. Click on your staff or missle weapon and cast another spell and then quickly tower shield and block again.

2. Tank wall in a doorway. Simple enough. The tanks either body block, shield block or else occupy a doorway so the mobs do not slip through and kill you. From behind this "wall", you can blast away. Good tactic here is a Firewall, Fog or Cloudkill just in front of the "wall". The mobs will instantly come to the fighters anyway and when they hold the line there, the fights will be quicker and smoother with less healing duty for the cleric.

3. Tumble, tumble away! Yes, it does make sense to put some points into Tumble for those times when a tough monster aggros you and you need to make a quick getaway while the tanks come in to save your butt. And tumbling towards and behind the tanks is always a good idea!

4. Keep immune items handy! When in some of the tougher dungeons, you can run into near instant hazards that must be avoided! A good example of this would be those dreaded Arcane Skeletons randomly tossing Cloudkill all over the place. If you know that the dungeon has enemy casters tossing Cloudkill around, it makes sense to keep a poison immune item equipped so you do not go BLAH! DEAD! when you run through a cloud! Same idea goes with places known for mummies and other nasties like spiders et al. Casters generally have terrible Fortitude saves so it makes sense to do a little preventative maintenance to avoid tragedies.

5. Try not to directly aggro undead spirits such as wraiths and spectres. Since undead spirits tend to fade in and out, it's usually a good idea to leave them to the tanks and clerics in your group. I have seen too many situations where a Caster shoots a magic missle or other blast attack at one, gets the aggro and winds up quickly dead from a succession of damage, drained levels and Constitution drain.

6. Know your monster! I know this is a generalization of sorts but the basic premise here is to have a strategy in place for dealing with different types of monsters.
Some Examples:
MINOTAURS- Be careful when randomly blasting these guys! Your blast will instantly draw their aggro and you will quickly find yourself on the receiving end of their next charge attack! This is where the tower shield and block idea comes in handy!
GIANTS- They are BIG! REALLY BIG! And "BIG!" usually comes with "Lots or Ouch!" for those with poor AC's and low hit points who are foolish enough to wander within range of their weapons or foot stomp attack. And a Caster knocked to the ground is a welcome target indeed!
KOBOLDS- Yeah, I know you can easily whack one or two with your staff or flame a few of them with Burning Hands but what happens when you are foolish enough to get yourself surrounded, huh? It's during these times that those corners in block mode look very inviting!
RANGED ENEMIES (any type)- There is nothing that a ranged enemy likes better than seeing a robed bull's eye running around the battlefield! He know they are relatively easily to hit with low hit points and they will soon be decorating the landscape unless they duck behind a tank's shield or corner for cover!