You know what I hate? I hate The One.
You know The One -- he's the guy or girl in a story who is fated to save the universe by dint of prophecy or lineage or genetics, or a giant plot-generating box the author hooked up to the electrical grid in his neighborhood which demands a fresh sacrifice every few minutes before disgorging awful storytelling advice. Like, Make your hero The One because he fits the details of a prophecy. Or, Give your hero amazing magical powers for no reason whatsoever. Or, Never explain how he manages to go from country bumpkin to Amazing Super Wizard Version 5.1 in just under forty thousand words.
I hate The One because there are no damn rules. Magic should not be one giant hand-wave. Magic in your book is an excellent opportunity for world-building and characterization: What does magic cost? How does it work? Power and its use should cost your characters something. In We Are Not Good People, I created a magic system that requires blood sacrifice: the more blood, the more powerful the spell. This simultaneously gives my characters a difficulty rating and also casts a new shadow on a number of historical events. Were they really disasters, those wars and terrorist plots? Or were they shadowy magicians requiring blood for their spells?
Like every good author, I steal little and I steal big. I learned to love a good magic system when I was a kid reading sci-fi and fantasy paperbacks. I can recall a couple that really made an impression on me:
Master of the Five Magics by Lyndon Hardy
I read this book as a kid, and the magic system Hardy creates remains one of the more interesting and entertaining ones out there. He imagines a universe that has (initially) five magical disciplines: Thaumaturgy, Alchemy, Magic, Sorcery, and Wizardry. Each form of magic has a clear set of rules that govern how it works. For example, Wizardry is the discipline that summons demons, and it has two rules: the Law of Ubiquity, which states that flame permeates all (making it a gateway between worlds), and the Law of Dichotomy, which states that once a demon is summoned it must either dominate the summoner or be dominated. All in all, a logical system that requires the protagonist to actually study and learn and think critically about the magic, instead of waking up one morning with the ability to turn people into newts or something.
Midnight at the Well of Souls by Jack L. Chalker
Chalker doesn't get name-checked often, but when I was a fat kid with a Commodore 64 and a tendency to wear glasses the size of the moon, I ate his books up. In his Well of Souls books he presents a simple idea: The universe is just a complex mathematical equation. Learn to see it and how to adjust the variables, and you can change reality. Essentially, that's magic, but magic with a system. You can wrap your tiny brain around the idea that if you could interface with the universe on a mathematical level, you could change any aspect of it just by tweaking the numbers, the same way you would reverse-engineer a piece of software.
A Spell for Chameleon by Piers Anthony
Yes, yes, Anthony is kind of creepy, loves his puns and the word "panties" too much, and if you didn't read your first Anthony book before the age of thirteen you will hate him. But the simple, elegant "one-spell-per-person" idea in the Xanth novels is very cool. In this universe, everyone in the magical kingdom gets exactly one spell they can cast, or one magical ability or trait. That's it. Some are tiny. Some are magician-level. What I like is that it doesn't always matter how powerful you are -- it's what you do with it. The main character of the first few books at first appears to have no magic talent at all, but in fact he's one of the most powerful people in that universe.
The Magic of Oz by L. Frank Baum
In this book, Baum tells the reader straight up that the magic word Pyrzqxgl will allow them to turn into any creature. But he assures the reader it is safe to print this word, because you have to pronounce it correctly, and there's no chance you can guess how it ought to be pronounced. Brilliant -- and certainly the unconscious inspiration for my own magic system.