Abjuration. The school of warding, protection and defense. Abjuration is a good, solid school. It has many good spells, options, is a master at counterspelling, and has been treated nicely by such books as the Spell Compendium and Complete Mage.
Specialize: Specializing in Abjuration isn't a bad idea, and makes for a good character concept (paranoid?). Focused Specialization (giving up 3 schools and 1 spell per level per day in return for +3 spells per day of that school) is a bad idea. I am currently playing a focused abjurber, and I must say that it was not a good idea. The school is too limited for that level of focus.
Prestige Classes: without a doubt, Initiate of the Sevenfold Veil (complete arcane) is the perfect prestige class for an abjurber. You essentially never die and have the ultimate protection with this prestige class, which is 7 levels long. You have to 9th level to qualify and there are a lot of feats to take though.
Taking levels in the Master Specialist will fulfill almost all those requirements though! Master Specialist is a good idea for fast qualification and some extra abilities that are somewhat nice.
Finally Archmage and Abjurbant Champion (normal spell progression but also good base attack, bonus abilities and a d10 hit die, perfect for fighter/wizards) are also good choices.
Spells: The following are the good, solid spells of the Abjuration school:
1st- alarm, endure elements, protection from X
2nd- arcane lock, protection from arrows, resist energy, earth lock
3rd- dispel magic, explosive runes, protection from energy, antidragon aura, reverse arrows, sign of sealing, magic circle against X
4th- lesser globe of invulnerability, remove curse, stoneskin, ray deflection, ray of deanimation, mass resist energy, wall of X
5th- break enchantment, dismissal, contingent energy resistance, lesser ironguard, indomitability, reciprocal gyre, refusal, wall of dispel magic, zone of respite
6th- antimagic field, greater dispel magic, globe of invulnerability, repulsion, aura of evasion, ruby ray of reversal, seal portal, greater sign of sealing, guards and wards
7th- banishment,spell turning, antimagic ray, energy immunity, ironguard, hide from dragons
8th-mind blank, prismatic wall, spell engine, wall of greater dispel magic
9th- imprisonment, mordenkainenís disjunction, prismatic sphere, effulgent epuration, maw of chaos, reaving dispel, unbinding, absorbation
Other Options: there are a few extra options for abjurbers, but they are not wonderful, really. A specialist Abjurber is a force to be reckoned with, and should focus on warding himself, his allies, blocking enemy magic, and using his non-abjuration spells to his advantage.