Depending on the setting, here are a few ideas!

1. If steam or cyberpunk-ish  consider a literal magic counter, like a pressure gauge. However, it does present the "I just read an rpg discussion forum" response from the reader.

2. A full body tattoo that slowly disappears as the caster's pool is drained. Good for ritual magic, and the tattoo could also be representative of important things to the caster. Might also consider at lowerr power levels, the wizard merely has a sleeve or backpiece, and more powerful magi might not have any more room to spare.

3. Opposite the above, a series of markings appear as power is expended.

4. Plain old physical effects would also work. Yeah, throwing seven fireballs might be tiring, but using a force-ram to knockdown a two-story would feel more like running the New York marathon.

5. Scanners, dbz style. Over 9000!!!

(sorry about 5, couldn't resist