To Special Effect Or Not To...

The first couple of times I describe the visual effect. In this case I'd point out the differences in the effect for different mages when they each first cast a spell. This would work best if the effect is somehow tied to their personalities, such as the Patronus in Harry Potter.

After the initial castings I'd only occasionally describe the effects, usually when it's important to know what effect the spell takes or for flavour. I wouldn't spend too much time on it but don't leave the reader wondering what a particularly cool spell might look like either.

I use effects for certain abilities. Healing, for instance, causes a warmly glowing light.

What this afforded me was the ability to say what was being done without saying it. Early in the story, I explained healing as a character "drawing healing energies" which resulted in a "dim golden light." Later, I was able to describe the same act by simply saying he rested a hand on the wound and then the light blossomed under his touch (or whatever fancy wording I used). So I got to say he healed someone without having to say "And then he healed [the injured party]."

I guess it taps into a show-don't-tell thing. Sorta.