How do you track several plot lines woven together? 

I usually run 2 or 3 plot-lines in parallel, so I detail those plots each in a separate document. But as the book goes on, it is sometimes difficult to make sure all those "threads" are woven together nicely because I have to manage 2-3 mini-books instead of one.

Any advice? I try not to rely on memory alone, because I tend to forget things I don't write down.