Reality Shift

What happens if while writing your novel, you suddently have a great idea for another novel?
Here goes all the rush you had over the current book, because the new book suddenly looks like a much better project to work on.

I have this issue with my role-playing campaigns. I'm starting to write this new, cool, unique campaign, spending a month or so thinking about it, spending another month writing it, and then BOOM! A new idea takes over and the old campaign looks boring and trivial.