One of the biggest problems I have when starting a new book is getting over the one I just finished. My mind will still be in the other book's universe/world and I won't be able to concentrate on the writing at hand. My advice for you is to take a break of writing for a while and think of the world you are going to create next. Don't be stuck in something you've already finished, it's not fun.
Step 1) STOP WRITING!! I know this might be hard, but just go on YouTube and watch adorable animal videos for nine hours. That's what I do.
Step 2) Start with the main character of your next story, and then create the world and other characters around them. Don't write it down just yet, keep it all inside your head.
Step 3) Keep your idea in your head for at least a week, and think of ways you could write it down.
Step 4) Force yourself to write it down. By this point, it will be terribly hard to write down this world on paper. Sorry.
Step 5) Go back to the ugly you've written and edit out the rough patches to make it sound better.
I hope this helps with some of the problems people have with getting over a book.
this is where i ramble; have fun