Memorable slogans are slogans that have built in mnemonics (devises that help the mind remember).  To remember the ways the mind remembers, remember the acronym PROPAR:

• P - primary - you usually remember the first time you ever hear or do something, the first item on a list, or the first couple of sentences of a speech or lecture
• R - repetition - obviously, we tend to remember what is repeated
• O - outstanding - you remember things that are very unique and unusual
• PA - positive association - we tend to remember new things if we associate them with past, positive experiences
• R - recent - just like how you remember the first item on a list, you also tend to remember the last item