Strategizing

Dave and I are working on picking the idea for our 3rd novel.  We have several ideas floating around in scifi, fantasy and military scifi.  Two of the ideas have been tested on a beta readers and have been received well.  We should be able to make a decision this weekend and then the fun starts.

Writing with a co-writer is challenging sometimes.  We have found over the course of 2 books that a detailed outline works well for us.  There's still plenty of leeway for spontaneity because as most authors know, characters have a way of sometimes writing their own plots.  

What works best for you?  Do you need a general outline, a more detailed outline or do you just wing it as you go along?