Writing a Backstory
Backstories don't necessarily have to be long or eventful. There's no need to write an entire book about your character before you even begin playing him. In fact, it might be good to save some of those story ideas for roleplay! The purpose of a backstory is not to make your character fully established, but to set him up so that he can be where he needs to be, so that he can begin his real story (in the game). The roleplay becomes the defining time in your character's life, like the events in a novel usually are to the novel's main character.
It's is not necessary to jam-pack your backstory full of exciting action. Indeed, some of the most effective backstories set up characters who thus far have not been involved in much excitement, and this might provide the motivation for them to look around for it - Bilbo Baggins is a classic example of such a character.
But certain events can help shape characters into the individuals you wish them to be. Traumatic events like the loss of a loved one, a terrible injury or a great disappointment can spur on both negative and positive character development. Fortunate events, such as the birth of a child or falling in love, can give characters something to live for. A spiritual awakening or a great revelation might also be the key to shaping your character.
If your character is meant to have a family or existing connections with other characters, their backstory is a great place to set up those relationships ahead of time.
Things to Avoid While Writing Backstories
Don't get too bogged down in writing every detail of every event. It isn't necessary to write a thesis paper on your character's life, especially if it hasn't been incredibly eventful. It's painful for casual readers to slog through paragraph after paragraph of laborious description when all they're looking for is a simple explanation of what's happened to your character in his life so far. When you start writing dialogue, you've gone too far - save the intense prose and write an actual story! It will be much better received. The backstory is simply not the place for that much writing.
Conversely, don't be too sparse. Even if nothing of note happened during your character's formative years, it's worthwhile to focus on a few significant events or whatever it is that your character latches onto when asked about their past.