I generally think of my characters as characters from a book, & thus tend to give them epic or odd backgrounds, something that would work in a story. Sometimes this can lead to tried-&-true fantasy tropes (the barbarian chief's son who left his village to explore the world beyond their lands), other times it can be a bit more complex (the assassin who was raised by a cult of insanely zealous monster-haters but who may be part-demon himself... or not, depending on whether he believes the cult or his insane mother who disappeared under strange circumstances after a rare lucid moment when she told him the cult had been lying to & using him), or other times a simple idea with lots of fun potential (the warforged who was found inert & damaged, & awoke with the belief that he's human - he just has a shiny skin condition). Even if a character was raised on a simple farm, I'll work in the idea that he always knew he was destined for greater things.
Another thing I've done is write the backstory in a style that fits the character (& often a couple of the first adventure summaries, though I don't usually manage to keep going with this). Thus my 'simple farm boy' character's story was written with Much Glamor & Drama, as befits one meant for Finer Things (even if such Finer Things were not yet aware of the manner in which Fate had destined them to be together, Fate being often rather lacking in communication skills) - tongue-in-cheek humor & a good dose of self-important styling. The barbarian's story, OTOH, was mostly told as dream-journey conversations with his grandfather, the shaman of his tribe. I never wrote an adventure summary for the assassin, but his backstory worked the dark angle of the conflict he felt between the beliefs with which he'd been raised, his own confused identity (who - or what - was his father?), & the recent strange encounter with his usually insane mother followed by her disappearance.
Usually I just write up a 1/2-page or so bit of background and jump in to the game, however -- the sad truth of my gaming life seems to be that no game where I am a player will last more than 8 game sessions.
