Literate Activity Spikes: Winter TV Programming Lull to Blame?

Happy News?

For the last two months, I've been sending a one-act to a bunch of festivals.  So far: one rejection, and one rejection with a request to see more work.  

This is the first play I've tried to have professionally produced, so I would (should?) feel pretty happy about that request... except it was written as part of an email.  I don't really know how personally to take that encouragement.  Maybe that theater company just has a very sweet form letter?

Oh well, I'm gonna go ahead and be happy about it, anyway :)  Does my bubble need bursting?!

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Have: novel rough draft.

Need: to weave in a subplot... figure out where that subplot fits in, and how it will effect what I've already got.  

Question: has anyone ever tried to plot their novel using index cards?  I've heard that it's a good way to outline a screenplay so I decided to try... but now I've got about a million index cards and total confusion.  Did anyone find this method helpful?  Did you do it in a specific way?