There is lots of sources of magic sapping fatigue. Some systems even have it set up where the mage gets exhausted with each spell/passes out when he reaches a limit.
There are various powers (particularly Invoker powers) that do damage to the character. How would you visualize those powers? Same idea; the character is spending their own vitality as fuel for the spell. Instead of having a human (or animal) sacrifice, they're using themselves. How would you describe someone being targeted by a Ray of Enfeeblement? Same idea; the character is being sapped or weakened by a source.
If you actually need a visualization, then the ritual caster grows pale and sweats terribly, swaying as he casts the ritual, because he's using his own current health to fuel the spell. Some hair might fall out, or he visibly loses weight if he uses the 100% method.
He could even use his own blood as a material component. And the loss of blood is represented by the healing surges - to the point the caster might faint if he goes all the way to 100%.