Magical systems
I tend not to get overly prescriptive when it comes to my magical systems. The bedrock principle is that magic *costs* -- working magic takes time, or effort, or pain, or sacrifice. Pretty much like anything worth doing, actually. I embrace most magical systems, especially in the urban fantasy series I'm working on now, where the premise is that *all* systems of magic work... if you can do them properly. But most people can't, because magic is complicated, and obtaining mastery over any particular type of magic takes a lot of time and effort. Most of the sorcerers in my books specialize. There are necromancers, pyromancers, biomancers, technomancers, oneiromancers, seers, chaos magicians, sex magicians, masters of sympathetic and contagious magic, and on and on. I pillage folklore and mythology and specious apocrypha for rituals, spells, techniques, and fold whatever seems interesting into the substance of my books. (My protagonist is something of an exception, as she takes a jack-of-all-trades, master-of-none approach to magic, which makes her more adaptable than most people she comes up against, even if she can't beat them head-to-head in their chosen specialties.) Making up a bunch of rules for magic doesn't appeal to me. It feels too much like playing Dungeons & Dragons or something -- and, while that can be fun, it's not really what I'm interested in when it comes to fiction. I feel like magic should be mysterious, weird, unpredictable, and deeply personal for the characters.