I'm feeling schizophrenic these days. In the writing sense. I'm revising one novel, and plotting another (for NaNoWriMo). But I've found a kind of synergy in this double-minded activity.
When I work on finding plot holes in my revision, it reminds me to keep that from happening in my new novel.
When I deepen character motivations in my revision, I'm conscious of working on the backstories of my emerging characters.
When I brainstorm plot points in my new story, I check to see if I did that effectively with my written novel.
When I plan research for NaNoWriMo, it forces me to check if I've done enough research for my previous novel to sound coherent.
Basically, what I do with one project, I try to do with the other. And I'm finding that both are becoming stronger as a result.
At the moment, I'm using three books in my revision and planning.
Manuscript Makeover, by Elizabeth Lyon is one of my favorites, and I'm finding her advice on revision just as effective in planning a new project as it is in reviewing a completed one. One of my favorite sections is her advice on creating backstory wounds for characters, and her "riff writing" exercises.
Blockbuster Plots, by Martha Alderson is a crash course in getting a new plot laid out that moves the story along. But I also like to consult it after the story is written, to make sure I didn't get off track in my enthusiasm.
Writing Fiction for Dummies, by Randy Ingermanson contains comprehensive advice. His Snowflake Method of planning a novel can't be beat. And he's funny, too.
So I'm plugging away with my brain stuck in two different stories. Are you single minded or double minded? Would it work for you?