How about taxing net worth of an individual? Calculated by market participants like they do today.<p>Tax 0,1% of net worth of those whose net worth exceeds say $10 million. Such people have to get assessment of their net worth and then pay their tax.<p>Of course they have incentive on doing slightly funny things with their net worth, but it is difficult to race them to the bottom in view of very large fines for crossing the line.<p>Of course people will skip citizenship then; but as long as they do business within country it is solvable too.