Well, we have some fun, non-standard ways...
Primitive Caster: +1 CL per extra component (V, S, or M) that you add to the spell; the components are specific, though. Still, Silent Image (shadowcraft mage standby) gets +1 from this, more if you have some cheap way of Silencing and Stilling the spell (just add the components afterwards!)
Elder Giant Magic: Get +1 to your caster level for every extra round you spend casting the spell; up to 3 rounds. Kinda sucks, though it is excellent for morning buffs. A Shadowcraft Mage imitating a Summon Monster spell could extend their casting time to the standard 1 round to get +1 CL, though.
Dark Speech: Incorporate Dark Speech into the Verbal Component of the spell to get +1 CL. Fun with Primitive Spellcaster, since you can get +2 CL out of adding a verbal component. You do take 1d4 Charisma damage, though, so better have an immunity lined up! (Planar Touchstone: Catalogues of Enlightenment (Pleasure Domain)).
Reserves of Strength: This one I don't even need to link.
Arcane Thesis: Restricted, but nice.
Spell Thematics adds +1 CL to 9 spells of your choice, and makes the spell harder to recognize.
If you can give the spell the [Fire] Descriptor somehow, there are a ton of ways to get quick CL increases there (Bloodline of Fire giving a +2 comes to mind...)