Mana Burn

Any time a spellcaster wishes to cast a spell, he makes a Caster level check. If the Save is failed, the spellcaster suffers “Mana Burn” from the eldritch energies coursing through his nervous system. This results in one point of temporary Constitution damage for every point the Save failed by up to the level of the spell he was attempting to cast.

This rule encourages upper-level spellcasters to make more intensive (and hopefully more creative) use of lower-level spells, rather than “throwing magic at the problem.”   And most importantly, casting spells now has an element of danger, perhaps even drama. Instead of a player merely stating that he wants to cause an earthquake, raise the dead, or destroy half an army, he now must worry about inflicting dire consequences upon himself by overusing such ruinous energies.