The Ninth Circuit "I plead the Fifth" opinion reminded me that people should plead other amendments more often.
When someone wants to stay at your house when he's visiting town, and you'd rather he didn't, you should plead the Third. (Special bonus if he's a government employee.)
When someone wants you to give him something for free, you should plead another clause of the Fifth.
When you're being told you shouldn't drink, and you want to, you should plead the Twenty-First.