In their book Made To Stick, brothers Chip and Dan Heath make the case for six factors (in combination) making the difference between what's memorable and what isn't.

Their six factors are:

1. Simplicity (any idea over one is too many)

2. Unexpectedness (a surprise grabs our attention)

3. Concreteness (the more dimensions of details the more hooks our minds use to create a memory)

4. Credibility (even untrue stories don't stick unless there's a hint of truth, such as beware of what's too good to be true in the urban legend that opens the book)

5. Incite Emotions in Listeners (we remember emotional experiences much more than anything else; we care more about individuals than groups; and we care about things that reflect our identities)

6. Combine Messages in Stories (information is more memorable and meaningful in a story form

SOURCE: The book Made To Stick, and Donald Mitchell’s Amazon review of the book