Not to hijack the other thread. Here's another, somewhat unexpected way, that writing short stories can help with your novels.This has been my experience with my current WIP. It actually started out to be a short story. Since I didn't have any brilliant short story ideas and I was writing it more for the practice anyway, I started from a fairy tale--Toads and Diamonds. Most of you have probably never heard of it and it doesn't matter. Even I have trouble finding the elements of the fairy tale in there any more.
First I deconstructed the fairy tale. I kept the family dynamic of the mother and two daughters. I ditched the fairy and brought in another character that I had played around with in another failed short story. (He ended up changing a lot.) I changed the gifts around so it wasn't what gift each girl recieved that made the difference, but how they used their gifts.
Things sort of morphed and grew as I worked on it and when I finished it was 30K words. Not a short story. So I set it aside. But I really liked the characters that I had created for this story and I wanted to give them a better chance. So, I would take it out and fiddle with it from time to time.
One morning I woke up and had the answer. I needed to tell the male character's story, too. Well, that was easier said than done. I tried several things that didn't work. I'd get a chapter or two and then it would just trail off.
Finally, by chance, I read a book that also has a broken main character (THE CURSE OF CHALION, Lois McMaster Bujold) and seeing how she handled bringing in the story of how he got to be broken, even as he was healing, gave me some new ideas.
Now I'm only one new chapter and the editing of three existing chapters away from writing THE END on the first draft.
I'm actually looking at another failed short story (15K words) as possibly my next project. That one clearly has a lot of story that takes place after the current ending.
So, you never know where a story might lead.