I have a few ideas as to good amendments.
Here's the first one. The idea is to attack the tax system at the root, by uncoupling government income from all other concerns. But to keep it palatable to the electorate, you have to build in the ability to raise taxes as well as lower them.
Section 1. A new legislative body of the United States of America is hereby empowered, to be called the Tax Senate. There shall be one Tax Senator per state, elected by the people thereof. The methods described for the filling of Senate positions in the 17th Amendment shall also apply to Tax Senators. The initial term of the first Tax Senators appointed upon the passage of this amendment shall be until the election year in which neither of the State's two Senators stands for election.
Section 2. The Tax Senate of the United States will concern itself only with rates of taxation. Its only power, delegated to it by the people, shall be to pass amendments to congressional laws of the form "Effective on <date>, the rate of taxation of <taxtype> on line <line-number> of U.S. law <law-specification> shall be <rate or amount>." Any other statement, bill, or measure passed by the Tax Senate shall be null and void. The Tax Senate shall assemble at least once in every year, for no longer than 28 days per year. The Tax Senate may not amend laws while not assembled.
Section 3. By the vote of the majority of members present, the Tax Senate may retroactively lower any tax rate in any law passed by the Congress. The lowering of tax rates is effectively immediately upon passage. By the vote of two-thirds of the members present, the Tax Senate may retroactively raise any tax rate, subject to signature by the President.
Section 4. The Congress shall have no power to pass a law containing any tax, except that the tax shall come into effect after the next beginning of the meeting of the Tax Senate.
Initially I had thought to replace the Senate with the Tax Senate, since the Senate is little different than the House after the 17th amendment. But doing that would require amending the text of the constitution all over the place. So I think just bolting on a new legislative body is clearer to write, even if it does the leave the anachronistic Senate hanging on.
The root problem that this hack to the Constitution addresses is the problem of the conflation of issues in the republican delegation of power. Practically speaking, it is impossible for the legislature to respond clearly to the will of the people on any but the top few issues, since individuals must be elected as representatives. This solution takes one particular issue, taxation, and "uncouples" it from all other issues. So the House and Senate would no longer be concerned with tax levels; and the Tax Senate would *only* be concerned with them. This makes the overall will of the people on the issue of taxation heard fairly precisely. (I have some faith that generally the people want lower taxes but that desire is being attenuated by other things.)
It would be possible to uncouple the politics of any given issue in a similar way, although you have to be careful to be able to separate the issue from other things. Perhaps we should have 1000 Senates for specific issues.
Now, taking Eugene's idea a little further, imagine the Tax Senate as above, except that instead of electing them via one man one vote, they are elected by one-dollar-of-taxes-paid-since-last-election, one vote. Now that would be radical! But too radical, I think, to stand up to Eugene's specificication that the amendment would not get promptly repealed.