"The reason flat and sales taxes are opposed is because they are deliberately designed to favor the wealthy.

The poor pay a major proportion of their salaries for necessities. The rich can divert a larger portion to investments or savings and have much more choice in what they buy. A sales tax savages the poor. For the same reason, a flat income tax (or one without a low-income exemption) hits the poor proportionately much harder than the rich.

When you add the accounting tricks the rich can afford to deal with the deliberately baroque complexity of the tax code, and the ability to write off losses on investments, the only real disadvantage they have is capital gains, and they are constantly working on reducing or eliminating that. If they had their way, they would pay no taxes at all, presumably because they are using every spare penny they have creating jobs for the ungrateful wretches that work for them."

They pay no capital gain by having Municipal bonds:
"Municipal bond is a bond issued by a state, U.S. Territory, city, local government, or their agencies. Interest income received by holders of municipal bonds is often exempt from the federal income tax and from the income tax of the state in which they are issued, although municipal bonds issued for certain purposes may not be tax exempt."
http://en.wikipedia.org/wiki/Bond_(finance)

It makes it a lower interest rate bond.

So, what besides flat tax would actually work to tax the rich?

Personally I think the best way to tax the rich is to make everyone richer. So, supporting a policy of high economic growth.
I don't see why taxing the rich is vaguely important.
But assuming it was important which country has ever been successful at doing it [I suppose murdering the rich could seen as a kind of tax]. So including murder which country has ever been successful at doing this?