The first question is what you want to achieve. Even the professionals forget this sometimes and they measure effectively meaningless things like "brand recognition."

For commercial advertising, the key metric is easy - higher profits. (Not just higher sales, although that helps.) Any campaign that doesn't pay for itself should be considered a failure.

For something like this - what's the goal? Is it to raise money? Change policy? How can you tell that the goal has been achieved?

It's not all that difficult to create striking and memorable images and words. It's much harder to do it in a way that has a practical effect not just on attitudes, but behaviour.