Do you have your kids memorize the presidents?
I don't. I want them VERY familiar with major events in US History & which president was in office at the time. We make a book of presidents by having them research each president and write a paragraph about each along with a coloring page on the facing page. I found whitehouse.gov very helpful for presidents' biographies and their coloring pages were GREAT! Each child does this over the course of a year or two as we cover American History for kids in grades 2-8.
My goal in doing this is that they'll find facets of different presidents to relate to and find some favorites. I think that will help them stick in their memory in a meaningful way more than if I had them just memorize them in order.
I think that if I really wanted them to memorize the presidents, I'd do it to music (the painless way to memorize most things for us) during their early years and then do the research in their mid to late elementary years.
P.S. As I was looking for a good example of a presidents song I'd let my kids listen to on youtube, they heard the songs, came rushing in from their school and asked if they could pleeeaaassseee learn one of the presidents' songs.