Past a certain stage, I personally feel you are better off using "absolute" effects to ward off blows, rather than trying to raise your AC, which tends to accrue at a rate slower than monster's to-hit rating scales (and are thus useless. The dragon with attack rating of +50 won't care that your AC is a hefty 44, for instance).
For example, mirror image/greater mirror image means that you cannot be hit by physical attacks, regardless of how low your actual AC is (mix it up with miss chances such as displacement to double its efficacy). Abrupt jaunt (PHB2) lets you teleport 10ft as an immediate action, useful for escaping a foe's melee attack.
Still, if you want to improve your AC, consider entering the abjurant champion prc from complete mage, since it increases the AC bonus granted by your abjuration spells. Sadly, the only armour-granting spell that is abjuration comes from BOED (luminous armour/greater), so the only real benefit is to improve the AC granted by your shield spell (which to your credit, can be cast as a swift action later on). Of course, this is assuming that you are not already using a mithral buckler.
Reduce person makes you smaller, which typically nets you a +2 AC bonus (+1 from being smaller, another +1 from the +2dex bonus). However, depending on your build, it may or may not be suitable.
Alter self into an appropriate form can grant you a decent NA bonus [popular forms include the troglodyte(+6NA), tren(Serpent Kingdoms, +8NA) and lizardfolk(+5NA)]. And if you can somehow access outsider forms, there is the ravid(+15NA) and dwarven ancestor(MM4, +18NA). If you are a construct (such as a warforged), XPH has the astral construct.
What is your character? This can help determine the suitability of buffs. Else, the list can go on forever.
