One reason that I'm revising my novel one narrator at a time is to help differentiate the voices. I don't think my three narrators sounded exactly alike before, but I was concerned that they were a little too similar.
When I started in on this second storyline, I thought I might have a lot of trouble switching out of the voice of the narrator I'd been writing for months and months. I worried that I'd given the first character too many of the traits that are important in the second character and that I hadn't left enough room for differences. The two characters are father and son, so a certain amount of similarity is to be expected, but even in my own head, they were merging together, so I didn't know what was going to come out in the text.
To my pleasant surprise, I only had to write a few pages before I started settling into the new voice. And while I don't have the distance to be certain, I'm pretty sure that this narrator doesn't sound like the other narrator at all. He has his own personality and his own perspective on the world, and I think the voice reflects that.
I find that it helps for me to keep in mind a few basic characteristics of the narrator I'm writing. These are big blunt generalities like "he is methodical" and "he has high expectations." I hope the text doesn't scream these messages, because the character's full personality is (supposed to be) more complex, but these bullet points help remind me how the character might react in a situation.
I've also been giving some thought to the specific vocabulary the narrator uses and slipping in some language related to his profession. Again, this is something that could be much too clunky and obvious, so I want to be sure not to go overboard. I'm even considering sentence structure and trying to create some subtle differences between narrators there.
If I can get all this right, I won't have to worry that my readers will ever get confused about who's talking.