Originally Posted by
Alleran
I'm personally fond of the concept that when casting spells, it takes a bit of your "willpower" or "inner strength" to get the spell out there (even if you're a prepared caster, just locking the spell into your mind takes some of your strength). As you cast more and more (or bigger and bigger) spells, you begin to drain your reserves. At which point, your body will start eating itself for the energy to power the magic. Drain away too much, and you not only age prematurely and become emaciated, but you might not have the strength left to keep your own heart beating, lungs working, and so on.
I think it's a fairly common device. I do know that the Shannara series has it (particularly for druid magic), and the Belgariad as well to some extent.