I can't do a full pullup either. A couple of weeks ago I couldn't even really do a chin-up (though I used to be able to). I just did assisted / negatives, which for me means... Jump! Then lower yourself down as slowly as you can. And jump a little bit less every time until you can do it without using your legs at all.
And then once you can do it from standing level, you work up to doing it from a dead hang somehow. I'm hazy on the details there because I've never gotten that far myself.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.