Look at the colors and theme of your home. You'll want to pick out a runner that complements that overall theme. For instance, if your home has Victorian decorations, you'll want a runner with intricate flowers and darker colors.
Decide whether you want the runner to extend onto the ground floor or whether you want it to start at the first step. If you have carpet on the ground floor, then you'll most likely want the runner to start on the step. If you have wood and need some extra rugs in the space, letting a runner continue for a few feet into the room will add a complementary rug.
Decide whether you want the runner to go down the hallway at the top of the stairs or stop at the last step. If the runner will go down the hallway, you'll want to properly tack the rug down to avoid wrinkles.