One of the most important things you must do when writing any story with fantastical elements is to make readers believe you as you step from the mundane to the magical. If you can convince readers to make this jump once, they’re likely to be with you the rest of the way in your stories.
If you write with a voice of authority, telling your story with confidence, you can get readers to believe almost anything. This is a reduction of a point I made a while ago that if you can explain something simple very well, readers will follow you more readily when it comes to something complex. In this case, if you write about your world with authority, nobody will question the werewolves who hang out at night in Fairmount Park.
The use of figurative language can help get readers to make the jump to the fantastical. Readers ought to be familiar with similes and metaphors (if they aren’t, one must wonder what they’ve been reading all this time), and these common devices can help ease them past reality. This is a technique that borrows from magical realism, where language that seems figurative at first can take on more literal meaning as the narrative progresses.
Another point borrowed from the craft of magical realism is the importance of having a reason for unreality. You oughtn’t to simply have magical creatures because you fancy them; rather, you must justify your departures from reality to readers. Show readers what the magic can give them in your story that plain old reality can’t. Alternately argue and charm readers into believing the unbelievable; if you do your job right, they’ll thank you later.
The final point to make is that magic must follow logic and reason; it must be like any other physical force. It’s not necessary that scientists/wizards make a study of magic in the same manner as they might study physics, but it should be plausible for them to do so. The wizards of the Unseen University in Terry Pratchett’s Discworld books do just such a thing, going so far as to split the thaum in a reactor on their squash court.
This dedication to the study of magic as a science is an extreme example, but it illustrates my point well. Magic that can do anything inevitably will, and that is both uninteresting and unbelievable. Readers will not be satisfied with a mysterious all-powerful magic, so don’t try to give them one. Better to give them magic which is limited, logical. You don’t have to give them all the rules, that would bog them down, and they may begin to be able to hear the distant rattle of many-sided dice, but you should know all the rules.