My plan is to buy all new entertainment books in ebook format first. Then if a book becomes a real favorite, or if for whatever reason I want to support an author more, I will buy a paper book version. I already have books scattered between three houses in three different states (mother's house, pseudo-mother-in-law's house, current apartment) so I really don't need more! I have such a hard time getting rid of things anyway. I tend to become unreasonably attached to bound stacks of paper with words on them.
Reference books and school books will probably be paper, I find it a lot easier to look things up in paper books.
(We'll see if this plan holds up, I've only had the ereader since Christmas. I already foresee two deviations, there's a series I have to complete.)