"No society can make a perpetual constitution, or even a perpetual law. The earth belongs always to the living generation…Every constitution, then, and every law, naturally expires at the end of 19 years. If it be enforced longer, it is an act of force and not of right.” —Thomas Jefferson (in a letter to James Madison from Paris, September 6, 1789)
Check out Sabato's 23 proposals here.
Update: Steve S. wants to know if we've read Jefferson's whole letter to Madison. Good idea.