LOL!!! We had a brand new student come into the dojo Wed. night, and said he remembered seeing one of our demos three years ago!
I can't speak to the effectiveness of demos, but I know our dojo hasn't had much luck advertising in local publications. Our best results come through word-of-mouth and through posting notices around the community (supermarkets, health food stores, book stores, etc.). I will say that my first encounter with aikido was when, fourteen years before I started, I watched a demo by another school (in Ft. Collins, no less, Jun) with my then-kenpo instructor. It made a strong enough impression that I remembered it and eventually started (one state and fourteen years later).