I try to make sure that my mages always have stoneskins active. The spell lasts for 12 hours, so there is no reason to not have it active always. Once you have stoneskin active, it should prevent casting failures at the beginning of a fight, allowing you to buff further once enemies are encountered.
Animate dead/Invisible stalkers I will cast whenever I enter a large area that I think will have a lot of enemies. They last for 8 hours, so no harm in bringing them out early. One of the greatest benefits of a single-classed cleric, mage, or a bard, is that they hit level 15 fairly quickly, at which point the animate dead spell will summon forth a skeleton warrior, strong, magic resistant, and capable of hitting almost any foe. Seen one do a critical hit on an adamantite golem for 32HP damage, not bad at all! And it's only a level 3 cleric spell to boot! It's a level 5 spell for a mage, but still is very much worth it, and it truly renders those level 5 elemental summoning spells useless (truly a relief, as I don't like sitting around for 3 rounds waiting for my mage to finish "mental combat" with the elemental, who sticks around for only a couple of minutes after that). Now, if only the A.I. would make the skeletons walk *in front* of the party while we travel the map, it would be great! The cannon fodder at the front, I say!
Any party member who has strength below 18 and will be using melee/thrown weapons, I will cast strength upon. The spell lasts for quite some time, and since I usually have at least 3 arcane spellcasters in my party, it's not a problem to memorize a few of those.
Blur/mirror image I will have my fighter mage using a lot, when he goes to melee. His AC is already very low due to the bracers AC 3 and dexterity bonus, and when combined with those spells and stoneskin, he's a tank in the true sense of the word. He can hold the enemies back and distract them, keeping other spellcasters safe, deal massive damage in melee, and, since he is nearly unhittable, he can do a bit more spellcasting while the enemy tries in vain to hit him.
I am now setting up minor spell triggers on my mages with blur + mirror image. These can be cast at will with no risk of a casting failure, and once they're up, it's unlikely that they will be hit by any phsyical attack while casting during the next couple of rounds. I'm probably the only one who didn't realize this until yesterday, blur improves saving throws! The spell description mentions that it makes it more difficult for offensive spells to target you, but it didn't specifically mention saving throws... oh well.
Bless + Chant could be combined in a minor spell trigger also, if you have a cleric/mage in your party. Allows you to get both of those short-duration spells fired off at once, and frees the character up more quickly so that they can cast some offensive/summoning spells, or more defensive/buffing spells if need be. I only use the trigger for really tough fights though, since I generally don't mind tying up my clerics with spellcasting, since fighting isn't what I have them there for.
Negative plane protection: Cast this on your tanks if they will be going toe to toe with vampires or other level-draining llamas. Not necessary if they have an item that prevents level drain, but there's only 2 in SoA that I know of, the amulet of power (only usable by spellcasters), and the improved mace of disruption. This spell doesn't last long, so you really need to wait until the buttheads are in sight before you cast it, as it's very irritating if the spell expires while you are still fighting them!
Haste is only good if you want to rest after every battle, as the characters will become fatigued afterwards. Improved haste will not fatigue them, but only affects 1 member, so is less useful. Hasting your summoned creatures might be effective though, but I haven't tried.
Spell immunity: Abjuration is good against buttheads who use the imprisonment spell. Immunity: Necromancy could be useful as well, for mages who have poor saving throws. I need to experiment more with spell immunity, I've not used it much since most mages seem to be too busy making circles around themselves to attack me with spells.
All of those other protections from spells and such I haven't used much, as my playing style has always been conservative, trying to rest as little as possible, and using those higher level spell slots for monster summoning spells seemed more attractive. I'll have to go and experiment!
Can't think of any other buffing I use. Protection from normal weapons is useless IMO, since any enemy worth buffing against uses magical weapons (except the Bhall in your dream sequence, it seems). Protection from magical weapons might be useful for a fighter/mage though. Time to test!
Edit: Oh yes, I forgot. I always have prot from evil 10' radius cast. Long duration, and it's a "cheap" spell, meaning that it's of a fairly low level and not on the same level as other useful spells I'd want to use. Chaotic commands I haven't used as much as I should, because it's "expensive". The AD&D system of spell memorization has caused me to compare the usage of spells with spending money! I really prefer the sorcerer's way of doing it. Set number of spells per level, saves headaches and messing around, but oh well...