First and formost be creative in your stamp- bank tellers tell me that they see numerous bills with one or two lines that all say the same thing- but when you have something different it draws attention to the bill and curiosity as to what the site is all about.
These are my personal views as it has worked well for me over the six years on the site.
You also have to change things up ever once and while as people see the same thing all the time and will loose any interest in entering the data we all request on getting a hit.
....... and one more thing stamp both sides black or red as worked for me.

[ message was edited on: 2010/05/25 ]