For me, usernames tend to be actual names. Not realistic modern-day names, but names a character might have in a video game, book or movie. For example 'Gralian' was the name of my undead rogue in World of Warcraft. Why did i start using these names as usernames? Funny story behind that; it's actually because i've always seen names you give to characters in a game you're playing or book you're writing always feel like pseudonyms for the player or author. In my case, anyway. This is because when i play my role in a game or write characters for my stories, the characters nearly always tend to be an extension of my psyche. They are manifestations of different parts of me, like they represent different values and opinions. They are the representation of my mental and emotional conflicts. [sup] No, i'm not crazy for having multiple personae.[/sup] and as such my online handles tend to be an extension of this. My online identity is... different to the real me in the real world. It's almost like we're two different people, and so my username reflects this - it's the name given to the 'other me', the shadow of the true self.
... Philosophising aside, that's how i choose my usernames. Pretty sure that's not how it works for most people, but at least you got a fun little anecdote out of it, right?