An example of alternating first person point of view done well is Will Grayson, Will Grayson, and in this case, there’s a simple reason for its success; the novel was written by two different people. The two characters who narrate the story are both called Will Grayson, but as each Will Grayson was written by a different author, the reader is exposed to two distinct voices.
This is not to say that an alternating first person point of view only works when there are multiple authors; however, it takes a well-practiced author to pull it off. Take Sara Zarr, for example, and her book How to Save a Life. The two protagonists, Mandy and Jill, are so different. Not just in their appearances and the way they dress, but in their actions, movements, internal thoughts, experiences, past trials…
In some of the novels I’ve read recently, the characters’ voices were so similar that I found myself constantly reading back or skipping forward to figure out which character was currently narrating the story. This happened even with stories that were told by characters of different sexes, and was particularly apparent where the narrators were of a similar age.
In some of these books, as if in anticipation of this very problem, each chapter had the name of the narrator at the top of the page. While this is certainly useful, it’s more of a cop-out than a solution, since the narrators should sound so different that the possibility of confusion should never arise.
Jodi Piccoult is another example of a writer skilled in voice. While there’s a consistency to her authorial style, she manages to create distinctive voices. Her book My Sister’s Keeper (told from 8 different points of view), exemplifies her mastery of voice and point of view.
The key lies in knowing your characters, hearing their voices in your head, understanding the way they think and the way they speak. If you allow your characters to narrate their story, their whole identity must be expressed through their narration. It goes without saying that a sixty-year-old woman will not sound the same as seventeen-year-old boy. Their use of language will differ.
Bear in mind that if your characters are not different enough to sound poles apart, you might have a deeper problem of character and characterization.