Practiced Spellcaster feat. Lesser Metamagic Rod of Extend (3000 gold). At 6th level, Extended Mage Armor = 12 hours. Two first level spell slots a day. You don't cast long duration buffs in combat
At higher levels, grab Greater Luminous Armor if you can, or Greater Mage Armor (Complete Arcane, 3rd level, +6 armor bonus to AC) if you can't.
Other good low level spells: True Strike and/or Wraithstrike. Great with Power Attack. Wraithstrike will be strictly better when you reach +6 BAB (so you'll want it by then), but True Strike can save your neck when you simply have to be able to hit something.
Another fun thing to do is get the Combust (2nd level) spell from SpC and a Spell Storing weapon. I don't see Spell Storing mentioned much for Gishes, but it's a +1 enhancement for basically an extra action every combat. I like it.