I would like each spellcasting class to use a physical item as a focus for using their magical energies:
druid/wizard/sorcerer - staff
paladin/cleric - holy symbol
spellsword/duskblade - sword
ranger - ring?
When a spellcaster learns a new level of spells, he must go through a ceremony with a higher level caster (trainer) to infuse his item with the ability to store the new, higher level of spells.
If for whatever reason, the focus item is destroyed or the caster can not hold his/her focus, the spellcaster can not cast spells.