The idea that when rehearsing something (in your
mind) that playing around with the idea, exploring it's consequences and forming ties with other information will make it stay in your head.
When memorizing Pi, for example, you might use maintenance rehearsal, repeating 3.14159265358979323 over and over again... But it wouldn't work well. Using elaborative rehearsal you can make the task easier by remembering that Webster's sed pi was 3.1416--he rounded up from nine. Also, there are 535 people in congress. All palindromes after the first one in this sequence have the larger number on the outsides, and 897 is my area code. This will last a lot longer. And this will work for anything you want to remember.
You can play around with any idea, and doing so will help it get into your Long-term memory and stay there.