[A federal balanced budget amendment.]
Because it's a terrible idea in practice. You need a degree of flexibility for emergencies, recessions, and war, otherwise, you'd have Congress sitting around debating for weeks, months, years over tax hikes vs. offset spending to try and pay for emergency relief or bombs.
Term limits haven't done jack for us here in California. Neither has a supermajority tax requirement.
But I would be behind preventing not just congressmen, but any government official from lobbying for any industry for five years or longer.
I'd also say we should force all meetings with lobbyists to be open door meetings on the floor of Congress, broadcast publicly.