Despite the original post being from 2005, BigB's answer is right on. I was not lucky enough to borrow a painter's and paid nearly 100 bucks at a paint store. But it had to get done, and NOW! So I bit the bullet and now we have one. Just have to keep some Goober from leaving it on a job. Works on roofs too. To use it with a stepladder may take 2.
We once (not on stairs but in a high lobby) stood an extension ladder straight up and 2 guys held it while I went up and changed a lamp...pretty good if you're in the circus...I'm not! You learn a lot about leverage doing that (and working one-handed).