Bringing Magic To Life

As an urban fantasy writer, magic is of course a big part of my books. I love writing stories about witches in particular and it is always fun to think of cool powers to give them. I have found though that when writing urban fantasy, magic needs to be believable. What do I mean by that? You need to be aware of the scope of that power and how it works. If the protagonist has a power that can wipe out an entire army with a wave of her hand, then it's not going to be believable if she can't take down a handful of bad guys because it suits the story.
Every power must have limitations. Whether that is what can be achieved with the power or the effect it has on the user. It could drain them or make them weak when they use it too much.
It must be consistent. Magic cannot be a get-out-of-jail-free card one minute and fail to work the next, not without good reason. How many times have you watched a TV show and thought why didn't the character just use their power to stop the bad guy? 
Take Charmed as an example - there are plenty of times when Piper could have frozen time to help the situation. Or Prue could have moved an object away before the bad guy grabbed it. You could simply say it was hesitation or they didn't react quickly enough, but often times it seems to be done to give the bad guy the edge, only for them to return later to be vanquished. 
There is a danger in making your character too powerful. It may be fun to make them the most powerful witch/being/magic user of all time, but what happens when they go up against the bad guy? The stakes aren't exactly high when you know the protagonist can defeat them with ease. 
Many storytellers resort to taking the protagonists' powers away temporarily. Or they are forced to create more powerful villains in each book for them to go up against. You can see in shows like Supernatural how this gets more and more crazy with time and they didn't even have magic on their side!
Plan your books out in advance, consider the magic used and how it will work against your villains. Some of the best ways of using magic is to have the MC come into their powers at the beginning of the story, which means they are not entirely in control of them until later in the series. You could make their magic unpredictable, where it doesn't work the way they want it to. Or you could make it so their power grows the more they use it. 
Magic is a great addition to a story. Make it fun, make it believable, and always know your limits.