I do bodyrows exclusively (3 subs=1pu) on D1 so I can keep going as it is for endurance. On D2 I do a combination of slow negatives and assisted pull-ups and try to get as much weight bearing as possible to help increase strength (at least I hope this will increase strength?). This makes your time seem really long as the slow negs are, of course, slow. I figure as I can't yet do a full pull-up then it really doesn't matter too much as to my time on D2 so I focus more on form and building up to a pull-up. D3 I've been doing bodyrows as 30 is a lot if you're trying to go for time (you do the same amount as D2 but on D2 you can rest between sets). I might try popping a few assisted or slow negs as I feel stronger into D3 but for now that's it.
As I said I can't do a pullup or chinup yet so take my advice with a big grain of salt! Some of the other guys have actually progressed through from not being able to do any to doing a lot. They threw some advice at me and from that I more or less figured this was the best plan for me. Most importantly it seems to be one I can at least stick to.
This guy goes over a few methods to work up to a pull-up http://www.youtube.com/watch?v=Ba7q14s5-2w
. If URL doesn't work try searching the evolution of pullups on you tube. He says if you can do a slow neg for 8-10 sec then you should be able to do a pull-up (the key word being should). I can manage almost a 12 second controlled slow neg and am yet to do a pull-up. Anyway, everyone is different in this regard I guess. There's tonnes of other info on this all over the net, but the main theme seems to be do a variation of subs and most importantly keep at it.