1. One perspective per scene.
Anchor your reader into the body of one character at a time. Why? Because readers want to be in the story rather than floating outside it. Whether you use first person or third person, show the world from one character's eyes at a time.
2. When switching POV's, start a new scene. This can be accomplished with blank lines, asterisks, or a new chapter.
Dean Koontz had two POV's in his book Intensity--the witness and the murderer. The witness stalks the murderer to his home. She's scared. But she still follows him, hoping he won't spot her. The murderer watches her follow him. Curious as to her purpose. Impressed with her resolve. Koontz masterfully weaves a heart-thumping, sit-on-the-edge-of-your-seat tale, expertly switching between the two perspectives with a new chapter for each perspective.Francine Rivers tells a love story in Redeeming Love. Husband--he's a God-fearing man--and wife--she's a prostitute since childhood--don't understand each other. At all. She's broken. He wants to fix her. She wants to run away. He does everything to stop her. Sometimes Rivers switches between POV's with a new chapter. Sometimes she uses a few blank lines.
3. Every character needs a voice of their own.
When writing dialog, you can portray non-POV's personality. Their individual voices should be heard. Your POV might be very straight-laced, but the side character might be sassy. Or the other way around. When the two characters interact, those personality traits should come out in the way they talk. Mebbekew in Orson Scott Card's The Memory of Earth never narrates the story, but you know what he is like. You know how he will respond to a situation because of the way he speaks and acts.
4. But not every character needs to be a POV.
Too many POV's in one book pulls your reader in too many directions. One of my biggest pet peeves about Robert Jordan is just how many POV's he has. (Another is that all females have the exact same personality.) No matter how minor the character, Jordan wastes space on two-bit players. The further you get in the series, the worst it gets because the cast grows. And you rarely get to know what's going on with the characters you love the most!George R. R. Martin also has a lot of POV characters. His A Song of Ice and Fire series has a great tale, and for the first few books, I was entranced. But by A Feast for Crows, I got frustrated. Every new chapter, I had to figure out who the character is again, where they left off, and why I should care. I had to reestablish a connection with the characters each new chapter, and for once, I didn't finish the book. Maybe I'll skim it when the last book comes out.
5. Characterize non-POV's through the POV's perspective.
The crazy part is that neither Jordan nor Martin needed to give everybody's perspectives. In Wheel of Time, many of the Aes Sedai traveled together. We don't need a chapter for each bitch in the pack.Furthermore, you can leave some secrets that are revealed later. Or you can use cues to give hints of what's going on in other characters heads.
More on this to come when I write about how to write from one perspective.