There are some stories that just can’t be told from one perspective—there’s too much going on, and no way the main character can be there to see all of it. That’s where multiple viewpoints come in—and where things start to get a little complicated.
The first thing to think about is how many viewpoints to use. The advantage to having a lot is that you can cover all sorts of different places, from all sorts of different perspectives or both—but that means making sure you have that many differentiable narrative voices, and you can’t lavish the kind of resources on each that you can on making just a few appeal to the audience. Having few lets you focus on the characters, but there are that fewer places it makes sense for them to be able to go, and things they could reasonably do.
Then there’s what each viewpoint brings to the story. The most obvious thing, of course, is location—one character will give you access to places that others can’t go, or won’t go, or simply wouldn’t be able to be because they’re over here at this time, which is necessary. But there’s also perspective—an adult will understand things a child won’t (or a child won’t understand things an adult will, and both have their uses), someone with a particular background won’t notice something that would be dead obvious to someone from a different background, and so on. If one viewpoint character isn’t giving you something that another does, you might want to think about whether you really need her.
How are you going to balance time between viewpoints? In some stories, it makes sense to include everyone equally, but if what everyone’s doing isn’t equally interesting, it might not be such a good idea to try for perfect balance. Consider also how to factor in who you most enjoy writing and how that might affect your balance; sure, a character might be enjoyable to write, but if she’s the one who isn’t doing much of anything good, you’re probably going to need to cut down a bit, or find an excuse to switch her with the one doing the interesting things that just won’t fit in your head quite right.
How do you plan on switching between them? Perspective transitions, handled badly, can be downright disorienting, particularly in first person where you can’t necessarily tell at a first glance who the narrator is. Some people deal with this by only switching at chapter breaks, and switching at every chapter break; some by labeling the beginning of each new section with its viewpoint character, some by doing one person’s scenes in first person and another’s in third person limited, some by changing the font, some by doing some or all of the above. Pick a method or set of methods and stick to it, if at all possible.
What happens if you’ve got a lot of overlapping time? While it can be important to see the same situation from two different angles, sometimes it can get a bit frustrating to the audience if it’s a fast-paced scene that ends on a cliffhanger, and they have to go through it twice without much advancement of time before they can move on to the part they want resolved.
Narrating through multiple sets of eyes is a useful element for a story; make sure you know what you’re doing and how not to go overboard.