"...The Constitution has a big red button on it labeled 'update'..." -- Scott Adams

I'd suggest a couple of changes for starters.

1. Term limits for members of Congress...four years for House members and six for Senators sounds good.
2. A federal budget must be created in January of each year and the spending in that budget cannot exceed projected tax revenue to the government for the year.
3. The salary of members of the House shall be $1 million per year. Members of the Senate shall be paid $1.5 million per year, both adjusted annually for inflation. Health insurance and retirement benefits for members of Congress shall be contributory and such benefits shall be the same as all federal employees as prescribed by the Federal Employees Retirement System.
4. A strict limit of $250,000 shall be placed on campaign expenses for any candidate for Congress. Such expenses shall be totally funded by the federal government. Candidates themselves shall be prohibited from expending their personal funds for campaign expenses.
5. Campaigning or the participation by candidates for both federal primary and general elections in media or privately sponsored events shall be limited to 90 days prior to election days.
6. Payment for campaign advertising or expenses by any political party, corporation, trade union, sovereign entity, or the candidate himself shall be prohibited. All such expenses shall be paid by the federal government.
7. Acceptance of campaign contributions, speaking fees, consulting fees or any income, in cash or in kind, beyond the salary for the office by any member of Congress shall be a federal felony.

I'm sure I could think of a few more improvements that could be made to the U.S. Constitution, but these would be good for starters.

Please don't tell me that some of what I've suggested violates the Constitution...that's what needs changing!