What is your character's motivation? If you take a few minutes to figure that out when you create your character it will make his or her back-story easy to figure out. Here are a few but I am sure there are some I have left out.
Heroic: It was apparent from the time he was young that he was destined for greatness. Even when over matched he seems calm and confident. No matter the odds he will be giving it his all. James Bond is a good example of a heroic character.
Reluctant Hero: Drafted into service by circumstances beyond his control. If he had his way he would be a humble inn keeper or farmer. It is only by grim determination and the need to protect those who cannot fight that he keeps adventuring. Someday, he will lay down his sword for the last time and live a peaceful life in anonymity. John McClane of the Die Hard movies is a reluctant hero.
Sidekick: Friendship to another hero is what motivates him. He may like money or fame as much as anyone but he will work for free if his friend is taking on a task. No task is too large and he will not shrink from impossible odds. Moonglum from the Elric series is the classic sidekick.
Duty: This hero has always seen it as his job to serve the greater good. He may not be as smart or strong as the other heroes but he knows what he must do. He can be cheerful or grim but he will not be frightened or bribed from his assigned tasks. Harry Dresden of the Dresden series is very much motivated by duty to protect his friends, Chicago, Earth and the Nevernever.
Fame: This hero wants to recognized for his talents. A statue would or a day in his honor would be a start. Someday people all across the kingdom will cheer his name and thank him for all he has done. The character Richard Rider (Nova) from Marvel Comics is very much looking for fame at least at the start of his career as a super hero.
Fortune: Your character wants gold, lots of it. His greed will win out if faced with overwhelming odds or fear every time. The Johnathon character from the Mummy film series always forgets his fear when he sees treasure no matter how much trouble it seems to cause him.
I admit I usually play the Heroic archetype. When I play Clerics and Druids in DnD they usually end in the Duty or Sidekick archetypes. The Decker in Shadowrun may start with Fortune as a motivation but circumstances on a run can turn him into a Reluctant Hero (happened to me). What do you like to play? Is it a combination or one that I may have missed?