I found when my imaginary world grew beyond one book that I might need some help remembering my own details. 

     This doesn't mean your story didn't make a lasting impression, even on yourself - this means your brain is working the way that it usually does and shedding minutae it is no longer immediately using.

     My solution to this dilema is to use a small notebook.  At each session I have it open next to me and I devote at least a page to every chapter.  I write each page number down that I complete and keep a count for that section.  If there are small details I think I might need I jot them down for reference.

     I also have been known to draw crude maps of locations so that when I write about that same place on down the road I don't totally change where the front door was by accident...oops!

     None of these things might work for another writer but they work for me and I developed them as I came along.  If I help whoever reads this in any way then I consider that a bonus.

     Happy writing all!