I have never been able to do even one pullup. Even at my best, I could not support my bodyweight when I had been boosted into a pullup position.
But I have recently found
Pulling Without Pullups? How To Train Your No-Tech Pull :: stumptuous.com and this has spurred me on to another attempt.
I am currently doing pull-alongs to start building some strength. My way to do this is to lie face down on my polished wooden floor at the top of the steps, and pretend the top step is my pullup bar. So I grip the edge of that top step with my fingers, and pull myself along the floor. The push back adds to the workout. If you have carpet that doesn't slide, try lying on a sheet of cardboard or something else that will reduce the friction. If you don't have a stair to grab, try using a sliding door track, or even place a broom handle across a doorway to simulate a bar.
Now, I realise this isn't exactly the same movement as a traditional pullup. However, I can feel it using the correct muscle groups, and I certainly know about it the day after. I consider it a bit like the pullup equivalent of the wall pushup - close enough to start generating enough strength that I should be able to then go on and get to the assisted pullup starting line.
Bifcus.