Have you seen the new forever stamps that the United States Postal Service has created?  This first-of-its-kind U.S. stamp is printed with thermochromic ink. Just touch the stamp to reveal the moon. Then once cooled, the image reverts back to the eclipsed Sun. The back of the stamp is imprinted with the path of the eclipse across the United States. 

I think that these are so cool and I don't think they will last long. For just $.93 (plus shipping) I went ahead and ordered some of the first day covers as a keepsake! You might want to do the same!