Having been burned a couple of times over the course of several years, I've implemented a system where a % is paid up front. This helps ensure that the client is invested in the project.
What have you guys done when you've been burned? (I'm sure it's happened to everyone, at least once).
Legal action (presupposing a contract was signed)? BBB reporting? etc.
Even though it sucks, it isn't so bad when the work isn't so specific that it can be used for similar projects in the future, but man, it really does suck when you've got 40-50 hours tied up in a project and absolutely nothing to show for it or an unresponsive client.
Once bitten, twice shy...