I've had the same question but I'm dealing with 6 main characters. I agree with what others have said - about certain characters taking note of different aspects of life, and different outlooks, and vocabulary.
I've tried to differentiate between my characters and I've tried a couple of things. Tying back to the whole vocabulary difference thing, some of my characters are more polite in their thoughts than others. Certain people cuss all the time, some occasionally, some never.
You could have a character make up words (if they are a mind-wonderer or unconventional thinker, maybe?). You could have some think in metaphors and similes, and others very literally. Maybe some notice all the negative things, some the positive (glass is half empty/half full).
I've also tried to play with sentence structure. One of my people eventually becomes an unreliable narrator, and I've attempted to implement a strategy I call "Yodification." Think of a 'normal' sentence of having the order 1 2 3 4. Yodify it, and maybe it's 1 3 4 2, or 3 4 1 2. It needs to make sense, but it's distinct enough to alert the reader of who's narrating. Not proper enough to be old-timey English, but not weird enough to make zero sense. Now maybe you don't go this far, but having a consistent system where certain structures of sentences are listed in one person as opposed to another could help separate them.
Also, maybe (based on personality), someone thinks a lot like this. They talk to themselves inside their head. Maybe they are sure of themselves and you rarely see a ? or an !. Just periods. Maybe another person is emotional and always has !!!!!! or ???. Maybe they are scatterbrained and you'll see dashes a lot (-) when they hop from one thought suddenly to the next. Maybe someone is a bit thick-headed, or super thoughtful, and they trail off with these: ...
Certain phrases could be common occurrences for one guy and not for another, as well (like "I don't know", etc.).
I've heard that if you get to know your characters, you will easily understand how they will react to certain situations and you will know how to write for them. Also, trial-and-error and feeling your way through it works, right?