For backstories, one thing I suggest is avoiding cliches or finding ways to work them in a way that isn't cliche. Things like:
1. He was part of a clan/squad/tribe and his/her whole group was killed except them.
2. An abusive childhood
3. They're the last of their kind
That isn't to say that these aren't good sources of backstories, I have a character or two that has the second one to an extent. The trick is finding a way to make them interesting.