Share 'How to Nicely say No Thanks'

I am sure you have all received books from authors and publishers or Requests to reveiw a book you weren't excited to read...   How do you say NO without completely turning those publishers and authors off from sending you books in the future?   or if your an author, what is the best "No Thanks" you have received?   KW http://www.ekfamilybooks.blogspot.com