These principles are what I use to create hard cookbook magic systems that fit the tropes of stock fantasy/RPG genres. I have several different settings in a shelf coming from different interpretations of these.
**Law I: magic means brains**
Epicious world: ambient mana accumulates in heads exercising intellectual faculties.
Dark setting: magic burns through mortal brain, destroying and re-purposing neurons.
DnD setting: sprites (spell slots) are given by fey as a prize for winning their mind games.
**Law II: magic means spells**
Epicious world: anomalies exist in the world and receive their names after being discovered; calling out these names summon them, the next step is to invest your mana into the summoned anomaly; magical names can be carved into metal powered by mana crystals.
Dark setting: magic formulas with certain gestures are thought to be "cheat-codes" from the universe the Architects have left behind.
DnD setting: magic words are "passwords" that tell one's sprites to go and summon some effect; they may be different for each individual.
**Law III: magic is religion-agnostic**
Epicious world: magic is treated as a cutting-edge science; some spells, mainly healing magic notoriously available to many kinds of priests but not to secular mages, require certain religious attunement of the mind to cast.
Dark setting: gods are widely believed to be dead since the opening of the gates of hell; practice of magic is often frowned upon.
DnD setting: all the spellcaster classes have different servant spirits (spell slots): wisps from the Nature for druids and hunters, guardian angels from the Heavens for priests and paladins, demons from the Hells for warlocks, ghosts from the Ancestors for shamans and sprites from the Fey for mages. Only the last comes with no strings attached, no higher authority imposed on the caster, due to neutral chaotic nature of the Fey.
**Principle of conservation of effort**
Magic can overwrite the normal laws of physics. There should be something to balance this effect, economically.
Epicious world: mana is the blood of modern society, and it makes certain aspects of it quite utopic;
gathering mana, whether for individual use, or in mana-crystals for use in machinery, requires access to open sky, therefore, access to acres of land; the less brains around, the more mana one can get; there's an economy to it, and also a nasty ecological impact; city mages live in high (and expensive) towers to gather mana before it rains on the commoners who can't use their little shares of it anyway;
learning spellcasting is an expensive hobby to have, and one that usually requires years of studying (to get a usable mana pool) and practice (to learn how to invest mana in each new anomaly you summon); most people and industries use magic in form of gem-powered artifacts, that's way cheaper than hiring a mage for routine work.
Dark setting: mages literally pay for spells with chunks of their self, sanity and mental ability; wizard towers are asylums for those practitioners who (inevitably) get disabled, but may still be very powerful.
DnD setting: all the spellcaster classes have different servant spirits (spell slots): wisps from the Nature for druids and hunters, guardian angels from the Heavens for priests and paladins, demons from the Hells for warlocks, ghosts from the Ancestors for shamans and sprites from the Fey for mages. Only the last comes with no strings attached, no higher authority imposed on the caster, due to neutral chaotic nature of the Fey.
A few of my settings break these laws: a space opera world, where it's called "psionic" ability and doesn't look much like wizardry; a low-magic fantasy world, where spells are based on tunes working hypnotically on the listeners, charming them, but don't really alter physics.