What I normally do is pack my mages full of offensive spells with one or two defensive spells at each level. In my opinion these are the best low level spells that you will continue to use through to BG2 therefore these should have you decently set-up for generic situations. I like to plow along until I hit a bump and adjust my loadout in such situations rather than micro planning.

Also in general I don't like low level spells that rely on saves to have any effect. My recommendation is to stay away from these until higher levels when you have Greater Malison and instant death spells.

Offense
1st Level: Magic Missile (best mirror image buster)
2nd Level: Melf's Acid Arrow (good damage and interrupt)
3rd Level: Flame Arrow (best single target damage)
3rd Level: Fireball (AOE fun made awesome if you have 3 mages casting at once)
3rd Level: Skull Trap
3rd Level: Dispel

Defensive/Buff
1st Level: Shield/Armor (only really need one I prefer shield due to Magic Missile immunity)
2nd Level: Mirror Image (a must have) Blur (pretty useful) Resist Fear
3rd Level: Haste (must-have!!!)
4th Level Minor Globe of Invulnerability, Stoneskin (I think its only in Tutu but its godly)

I don't pay too much attention to cleric and druidic magic. But I would focus on exploiting their buffs. Having one or two of each buff memorised and cramming mostly healing spells has always served me well.

It does seem to speed up spells cast by wand though

That's because it counts as item usage, not spellcasting

Just to add my 2 cents:

In BG1, focus arcane spells on disabling spells (sleep, web, horror, etc). Mages can make the game very easy in this fashion - as support for the muscle.

Direct damage spells don't become really useful until later levels. Do NOT bother having low level mage memorize magic missile to toss out a measly 1d4+1 when he could throw out a sleep and disable a field of baddies.

BG2 is more interesting, as there are many different play styles/strategies regarding spell choices.

An effective way to get off party unfriendly spells is to make use of invisibility/improved invisibility/hide in shadows to scout ahead and when you find a convenient clump of enemies. You send a cleric and a mage just outside the fog of war to cast 2 area of effect spells at the edge. For example entangle and stinking cloud then maybe follow it up with fireball and summon skeletons since the enemies are all trapped by the cloud and the web and the skeletons are not effected by the cloud and can rack up alot of damage on the guys caught inside even if some of them get randomly held.

Scouting ahead is useful in general because you can tailor your spells and overall strategy to the fights ahead in a roleplay friendly way.

Building on this the kitty cat familiar is one of the best scouts in the game, as it has good points in hide and move silently, is magic resistant, and is hard to hit. I often use it to help me draw enemies from groups to pick them off.

I really believe the best way to defend from a punch is not be punched at all! What has worked for me is having my cleric (was using Branwen, but she ditched me) puting a mob of undead or wolves ahead. Even if they don´t damage an especific target, such as an vampiric wolf, they serve as target to the wizard or other minion and hold them back while archers, entangle, stinking cloud, whatever, put the guy down. I was playing just with Imoen, Branwen and Mainchar as a mage, it got almost easy in certain spots. You can almost feel the despair of that poor monster, when it just thought it was almost putting the last undead to shatters, some blonde broad summons a pack of war dogs just behind him.
But need help on something: ludicrous setting, I´ve casted the shapechanger spell while inside Durlag´s Tower and now I´m stuck! I´ve become an ogre and can´t go through those tiny doors anymore and the spell don´t fade away!

If you don't mind sullying your morals you can just use ctrl+r to heal your character (needs debug mode on) - also removes all spell effects, both positive and negative. Should set your pc back to normal

Magic Missile is great against high AC (well, low AC, but lots of armor -- you know the drill) creatures b/c you get an automatic hit. 4 hit points is great for a turn of complete misses.

I got through the game with charming creatures and people. Every item I could find to help me charm was used up. And I almost always had some charms in my spellcasters' memory banks.

if you think BG 1 spell use is hard.... :)

Web is a good spell but, in BG 1, has the nasty habit of horribly ensnaring your party members (if they aren't protect w/ a Freedom potion/spell/item). For some reason in BG 1 (non TuTu) Entangle does not seem to affect party members so it is a slightly better spell to start out with [Jaheira has it when you get her] for CC.

A good trick to learn is to use Invisibility on your mage (combined w/ an item of Non Detection [there is a cloak available quite early in the game... if you wish to be slightly mean and not return it to its owner]) and have them sneak up to cast a Fireball/Stinking Cloud on a group of baddies is fun [combine that w/ Entangle for some good giggles].

Magic Missile is often underestimated, but the low cast time + increase in missile # w/ level + ability to interrupt [and bypass most DR - Magic Resist is annoying though] is a must on lower level characters.

Another I like using is Chromatic Orb - it does different effects based on level, and higher levels have a Death effect [instant death on enemy it hits who doesn't make a Save vs Death]. Laroch's Minor drain is a nice one that has a side effect of Life Steal if no saves are made.

There is a ring hidden around Friendly Arm Inn that is a great boon to spellcasters - it is called the Wizard's Ring (iirc) and doubles the amount of 1st level spells a mage has available to cast [eg: instead of having 2 to start w/ you have 4 - each time a level up grants you another spell slot the ring gives you one more so you effectively get 2 on those levels].

It is quite difficult to find (Especially with the BG 1 engine lacking an Item Highlight key) but well worth it if you do.

^
I

And for those who like to roleplay, talk to a nobleman on the 2nd or 3rd floor of the Friendly Arm Inn, as he'll actually tell you about how the magic ring got there. Then you can go look for it yourself, with the hints given to you.

And before that, when you meet the friendly old mage at the beginning of the Coast Way map (the day after Gorion dies), certain dialogue options with him will tell you about a magic ring a little north from where he talks to you.

Yeah I too noticed Entangle doesn't affect my party members in BG1, whereas it does in BG2. IIRC, Entangle DOES affect party members in BG1 when using the boxed games.

I own both the boxed games for BG1 & BG2, and I own the downloaded copies of those games from GOG here, am currently playing the GOG version of BG1, and I noticed that not only is Entangle different from the boxed games, but spells like Chromatic Orb and Sanctuary have been changed very much from the boxed games as well. Does anyone know if a mod is what changed it (as I have a few installed) or the changes to these spells are from the GOG copies? Either way, I like the improvements. No reason a cleric can't cast Sanctuary on party members, as was the case in my boxed copies of the games.

(SPOILER)

Along the same lines of roleplaying your way through the game, if you choose to help the gnome Galtok, on the map where the Nashkel mines are, he'll tell you of his magic wand that the kobolds stole from him. Its a really nice wand to have, especially for a 2-4 level party.