A vacuum. A little red vacuum that you can vacuum the stairs with. We found one in a car so we kept it and now we vacuum the stairs with it. It's pretty handy. Not very noisy. But if the little thing that catches all of the dirt flies off in your face it's disgusting. So don't let it overflow. Or just constantly watch it. They might come in other colors or something, but I don't know.
The dirt devil is on a chair right now, and I'll probably use it to vacuum the stairs later.