Before I started writing this book, I spent a lot of time thinking about magic. The book is paranormal, so there is magic ... and as I thought about it, I realized that I have strong feelings about magic in books. So keeping in mind this is just my opinion, here are the "rules" I unofficially developed for using magic in a fantasy:
1. It must have a cost. This is a BIG ONE. Strong magic shouldn't be effortless, without any strain or cost. If I wanted to get all literary, I'd say that magic is actually a metaphor in literature for strength, and strength of any kind often requires development. Monks aren't born meditating; body builders aren't born bench-pressing twice their weight; and Olympic sprinters aren't running a 100 yard dash in a blink when they're ten. In each case, the potential is there, but it requires work and sweat. So it goes with magic. I think it should require something of the users.
2. It must have limits. I read that JK Rowling's greatest challenge with magic was deciding what WASN'T possible. Three cheers for that. Impossibly powerful magic is either boring to read about because it's impossibly powerful, or the author is cheating by not using the magic to its full potential. If the hero can lift 100 million tons in one hand, why pretend he struggles with 10 tons at a crucial scene?
3. It must have a system. This is harder to explain, but I'll try. I generally dislike magic that simply IS. I like magic that has a system of rules. Again, this mirrors real life. Ultimately, the magic has to come from somewhere and there should be discrete steps that are used to make it happen. If these steps aren't followed precisely, the magic fails. In a way, this relates to #2, because the system imposes limitations. If a spell takes 10 minutes to cast, that 10 minutes is a limitation (and huge potential plot point).
4. It should escalate. Again, just my opinion, but in a paranormal book, it's important to first establish the world and set up the story. Magic should make a slow entrance, because you first need to convince the reader of the authenticity of your world. If you drop magic into a normal setting, all at once, then obvious questions abound: "Why isn't someone calling the news?" "Why is this the first time that's ever happened?" etc., etc., etc.
5. The reader learns about magic at the same pace as the characters. Many books with magic are actually, subtly about a magical EDUCATION, in which the character slowly learns the cost, the system and the limitations of magic, gradually moving from simple spells to more complex spells. Again, like real life. Mastery is only possible once the reader and the character have learned all their lessons the hard way and earned the right to use magic.
6. Finally, magic is only a tool, not a value. Magic isn't necessarily inherently good or evil. It's like electricity. The same power that we use to fire up our computers and brew our coffee is also used to electrocute prisoners to death and deliver shocks to the genitals in Third World dungeons. So the point of the story isn't the magic itself, but how it is USED, which is actually a reflection of character.
The thing I ultimately realized was that magic is cool, but it's also a literary tool that reflects the acquisition of any knowledge (and growing up). Arthur C. Clarke said that any sufficiently advanced technology is indistinguishable from magic. So when I'm writing about magic, in a way it's the same as writing about the construction of a nuclear reactor or space ship—it's about discipline, ambition, study, sacrifice, and finally mastery. But more important than all this, in the end it's about the choices the character makes with his or her power, which is really the same dilemma we all face every day.