I'm nearly finished outlining a novel (this will be the second completed novel outline with a third starting in a few weeks). The first novel was outlined using The Marshall Plan. The current novel outline is being put together using the plan from First Draft in 30 Days. (The next will be outlined using the plan from Book in a Month, as a means of comparing the various methods.)
During the outlining process, one of the most difficult things for me is keeping track of all of the plot lines. That's the main plot, the hero's/heroine's subplot(s), any romantic subplots, and subplots of additional characters. It's easy enough to set the goals, and even work out how I want each character to get from point A to point B. The tough part is working it all in within the chronological order of the story as a whole, while remembering not to ignore any one plot line for too long.
One of the best things about playing with these various novel outlining plans is that they approach it differently. They don't always work for me as the authors intended, but they let me find out what I'm comfortable with - all methods can be adapted to fit your work and writing habits.
For me, the key is to plan out my plot lines and viewpoint characters for scenes before I write or even outline the scenes - In other words, if I know I'm going to have 50 scenes, I can map out beforehand who the viewpoint character will be for each (and the corresponding plot line) before I have any idea what the scene will actually entail.
That might seem strange to some, but it's perfect for me. It forces me to think about characters I might otherwise forget to focus on for a while. And I don't find it rigid in the slightest. If, when I'm mapping out details in a scene, I realize the story couldn't logically move on to the next viewpoint character in my scene list, I can simply re-arrange some things or add a scene to bridge them (that's the beauty really - once you know they should be a focus somewhere near there, you can move them around a bit without letting the story line go stale).
I'd love to hear how others track the subplots / story lines in their novels through the outlining process. Do you simply outline it, and then go through to track the plot lines to make sure everything's resolved, or do you prefer to lay things out a bit more up front to guide you in brainstorming scenes through the course of the story?