Quote Originally Posted by Alleran View Post
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.
I was never really too crazy about that. I understand that in some stories it's important so that the wizards aren't overpowered and able to just end the story right there - but I just don't like the aging, draining life part. A system that I liked very much was in BESM d20. Every character (regardless of race/class) had Energy points. If you cast spells (or other certain abilities), you'd spend energy points. If you ran out of energy points, you fell unconscious. So, a similar idea, but more of a temporary issue than sacrificing your long-term health. It allows the magic to be more accessible but still keeps you from going over the top in certain situations.