Usually what I do is have an overall plot line with certain events marked on it that will happen at particular times. It gives me a linear plan of the book so I can see how I'm going to get from A to B (whilst wandering through C-Z on the way!) The subplot I have on a separate line to start with and then I merge the two so I've got a novel plan to follow.
Once I've got that, I break it down into 'chapter' chunks. I write down what's going to happen in that chapter, how the characters might react - any lines I can think of that might be said, and any little bits that I think might be handy to include. Pictures and ideas are stuck on the planning as well.
It's not as rigid as all that - some bits get thrown out of the window and other bits get added as I write. I also don't stick rigidly to a 'chapter' being a chapter. Sometimes they morph into 3 chapters, and sometimes I find that there's little to say.
It's a great backup to know that the planning is there, and it does stop you from stuffing up your plot! It's also handy in terms of pace because you don't spend too long on any one area ~ usually.
That's me anyway.
Imogen