Good question, Jason. The proper use of magic is one of the trickiest aspects of writing fantasy. If badly done, it can easily unbalance a book.
In my case, one of the things I did was go back and reread the Master, J.R.R. Tolkien. Virtually all high fantasy written today, including the work of most of the authors in LEGENDS, in heavily influenced by Tolkien.
Rereading LORD OF THE RINGS, it struck me very forcefully that Tolkien's use of magic is both subtle and sparing. Middle Earth is a world full of wonders, beyond a doubt, but very little magic is actually performed on stage. Gandalf is a wizard, for instance, but he does most of his fighting with a sword.
That seemed to be a much more effective way to go than by having someone mumbling spells every paragraph, so I tried to adapt a similar approach in A GAME OF THRONES.