Writing a Sequel

So, I am currently writing the sequel to The Ridge.  It has been a very different process than writing the first book.  It is fun because I can expand on story lines I began in the first book, but I am also running into some problems.  I never anticipated the consistency issues.  I keep having to go back to The Ridge to check my facts about character ages, places, etc.  Obviously, the facts have to be consistent from book to book.  For instance, I just realized that one of the characters is six years younger than he is supposed to be.  I guess I just found my first big revision project!  Ha!  If anyone out there has written a sequel and has some tips about how to keep those facts consistent, I would love to hear from you.