I find myself more and more impatient with those who tell me that magic in fiction has to have rules, and that fantasy writers have to know them. Magic has to have limitations–it can’t be an easy solution to, well, anything–but that’s not the same thing.
My rule of thumb (which of course I discovered only after I was already doing it) is that magic is the thing that gets my characters into trouble, rather than out of it.
I don’t want my magic to be trustworthy and bounded and tame. I want it to be messy and unpredictable and troublesome and awe-worthy.