A character's backstory is a great tool from which to draw for both GMs and Players; it is also a great way to engage everyone on more levels of the game. So, why is it that we settle for such poor unoriginal stories? Why are all the parents dead? From where are all these orphans coming? It's a real epidemic, but it helps explain the lack of friends or enemies. These characters were never properly raised and don't know how to interact with other people—what else could explain their behavior?
Next time you start a game, tell yourself and your players that you will not settle for mediocre characters anymore. “Because you always wanted to” is not a good enough reason to join the militia. “Because your parents forced you” is not a good enough reason to pursue a career in magic.
These characters often have nothing to lose but their possessions and their lives and they risk all of it every day. Why is that? Why are they willing to put their lives in so much danger? Money is not a good enough motivation, and neither is depression due to the loss of family. What is the goal of the PCs?
Try this, make a list of things you could use from the backstories of your characters: people, events, places, items, etc. Take that list, and use everything on it.
If your list isn't as long as you'd hoped, then next time you run a game, ask for something more acceptable. Make your players flesh out your world for you. Ask for a friend, an enemy, a place, a meaningful event, and a special item, and make sure every PC has one in his or her story that links to the PC in some way. If you have a party of four, you have twenty plot items for you to abuse.
A childhood friend, a pen pal, a friend from work, a close cousin or brother, etc. This person means a lot to the PC and should have a large impact on the PC's life.
A childhood rival, an evil twin, an evil landlord, etc. This person has caused deliberate harm to the PC on more than one occasion and causes the PC's blood to boil at the mention of her or his name.
Home towns are too easy and will not do. You need something less obvious. A first dungeon, a mysterious island that the PC stared upon as a child, a town to which the PC cannot go for fear of being hanged. These are more exciting and can provide more fuel to a smoldering game.
A Meaningful Event
This is something that has changed the PC's perspective on life and has led the PC to where she or he now is. This is something that haunts the PC's dreams. Births and deaths are easy, but acceptable. The death of parents is not acceptable. This can be a future event envisioned by the PC in a dream, or a massacre the PC took a part in, or an earthquake that swallowed a mountain. Something powerful with a bit of mystery sprinkled in.
A Special Item
This is something that the PC and others revere. Perhaps, it is a family heirloom or the ancient hero's sword that the PC now carries or the necklace the PC's daughter made before the PC left for good. Whatever it is, it's important.