Go to a dollar store or a WalMart and buy a bunch of those cheap-o 1/2 inch binders, or several very large binders, and a package or three of those divider pages. Dedicate each 1/2 inch binder, or one section of the large binder, to one idea, and purge your brain of everything to do with that story on either looseleaf paper, graphing paper, blank sketching paper, or typed on the computer and printed. Mark the next 1/2 inch binder or section of a large binder for a second idea, and purge your brain of everything to do with that story in that section/binder. Continue doing this until you've purged your brain of every story you've thought up.
Then, make an index, label the binder(s), and continue to write whatever comes to mind and put it into each section. This will help you stay organized and because you've gotten everything out of your head, you can clearly focus on one story. If a change for another story pops into your head, or a new idea for a new story comes up, purge, organize, and get back to writing. Hope this helped!