Depends on what you want out of magic. There are several different ways you can do magic, all perfectly valid. I find the most successful kind I use tends to be a supernatural force that is constrained only by the physical limitations of the user. In general, though...

[b]#1:[/b] The more powerful the spell, the longer it should take to gather the energy to complete the spell. Compare to, say, a download. If you're downloading a small movie clip on YouTube, it should be done pretty quickly. If you're trying to download an entire movie however, that will take a little longer.

[b]#2:[/b] A detection method for mages. This can vary from mages being able to sense other mages to some physical change in wielders of magic that makes them visible to something else. This prevents "gods in robes" from forming, which are characters whom are potent magicians and can plainly and obviously defeat any character they come across simply by being able to hide their talent. This would be like if a professional swordsman could hide his two handed claymore and then just have it magically appear in his hands.

[b]#3:[/b] Physical curtails. Whether this is a hard limit in which a character cannot cast spells beyond X level of power or a soft limit in which characters begin to hurt themselves casting beyond a certain point or level of power, there needs to be something that stops mages from downloading meteor.exe and wiping out the entire enemy fortress. Single handed.

[b]#4:[/b] Outright banning certain forms of magic, maybe because it's outside of human physical capacity, maybe because magic simply doesn't cover it, but things like chronomancy--power over time--and unlimited no restrictions shapeshifting, or mind control at any range, or stuff like this, is absolutely impossible to counter or balance without rigging even more broken circumstances to fight it with, which upsets everyone involved and puts into question the true power of magic in your universe.

Beyond those four points, there's not much that needs to be said about magic, because magic is quite literally anything outside of what is normally impossible made possible. Like summoning fire and not burning yourself as a result.