I put this on the back of some business cards I printed up:
Finally, a way to use your vote that can actually make a difference.
The existing minor parties feebly attempt to win at a rigged game. The major parties have perfected corruption, and the one thing they still need from us is our votes. The Protest Party is designed to wrench power back from the corrupt system by holding our votes hostage until our anti-corruption demands are met.
United against corruption and wielding the power of our unified votes, we will set the major parties against each other by demanding that they take action - or lose elections.
I've found I get some eye rolls at the mention of starting a new party. This message establishes common ground with these skeptics of minor parties and explains why the Protest Party will be fundamentally different. That's the idea, anyway.