Safisan definitely brought up some good points there. I'll use ADV is an example been as I think it is definitely a good one when looking at keeping roleplays going. I don't just say that because it's my idea. Recently, I've had 5 players drop out. The roleplay is a trilogy and involves 4 gangs that until recently consisted of 17 players all together.
I tried it once before under a different title and it died epically. I was never going to redo the idea really until one night when I was talking about it to someone who had been in the original, that wanted it redone and other people gained interest.
So I started with an interest check. One thing I made sure though was, that my roleplay had a clear beginning and a clear ending to work towards, as well as points to get to in the story. I also needed enough players. I originally when for 4 in each gang and then rogues, then it turned out 3-4 people.
The first thing I came across really was the South gang's lack of posts and where they were heading. In the end, players dropped out and so I've now had to work the South Gang's death and demise into the roleplay, which has worked fine. Then, I've had to deal with another character been killed off as well as a character leaving RPG, an important one which I've managed to find another user interested in playing.
My next challenge was how to get around 17 users posting. My first thought was post order, but then I thought, that users would then be waiting 17 turns to post, and that could be a while, with time zones, especially as everyone in the thread are world wide. So, the way I got around this was 2-3 posts per day.
My hardest task yet is keeping players entertained, especially as they watch others die and drop out. Luckily, the nature of the idea means that players can afford to die and be part of the story if they wish to drop out. I've made sure to emphasise that the players use the OOC and each talk to their group and form smaller inside plots to work towards.
North gang also came to a stop for over a week, which I thought would completely die, but again its taken off. I think the only reason all this has happened though is because I've managed to collect a group of dedicated roleplayers, which I know is rare. I've created a lot of roleplays and so far this is the only surviving one in terms of groups.
Personally, I think the black book idea is a little wrong. Just imagine if you were on it and no one would accept you into their roleplay, because of it and the reasons you had dropped out, were valid. Instead, maybe it's worth looking at users histories with posts before accepting them.
Now, that's just my two cents on the whole thing, and an example from one of my roleplays going in an attempt to help people on certain aspects if they find the roleplay deteriorating. Another thing to do, is in a roleplay you'd joined, constantly add to the plot line with new and exciting ideas to try and help the plot along. I always encourage this from my players ^__^.