A national sales tax is a) regressive (disproportionately hurts the lower incomes), and b) detrimental to GDP.
A line item veto is counterintuitive with respect to the Constitution (i.e., it crosses the separation of powers line).
The short answer to the question in your thread title is to get the economy back on track (revenues will increase as a result). However, there are other fundamental issues that need to be addressed regardless:
1. Gov't revenue as a percentage of GDP, currently horribly too low (about 15%), seems to be low historically (say, 20%) compared to other 'first world' countries. This needs to be looked at. Perhaps once state and local taxes are factored in, the resultant 'national' tax rate is more in line with other prosperous countries. I doubt it, but I haven't looked at combined figures.
2. Given a studious effort, government efficiency might be increased by roughly 10%, but this is pretty much peanuts ($350 billion). You could -- and should -- cut the defense budget in half and save more than that ($400 billion).
3. Entitlements, welfare, social security, it all is what it is. You can't cut it without damaging the economy, and you should not cut it regardless. Again, an increase in efficiencies might help, but it would be peanuts. You could do some workfare if it made you feel better, but it wouldn't impact expenditures to any great extent.
4. Gov't revenue needs to go way up, as noted in #1. I think the transaction tax on Wall Street should be implemented.... as it stands now, I can buy and sell a billion dollars of stock a day and not pay a dime in sales tax (actually I buy and sell a bit less than that, and not every day
). Total annual dollar volume on the NYSE and NASDAQ combined is roughly $17 trillion... a 6% federal 'sales' tax on that would reap $1 trillion in annual revenue. That, plus halving the Defense budget, would pretty much balance the budget right there (although the defense cuts would have to be made delicately to avoid GDP impact).
5. Foreign aid and such is peanuts (what, $80 billion?). Plus, like most rich people, we generally have to buy our friends. Although all the war crap needs to stop immediately, as slashing the Defense budget (above) would require.
6. Something could be done to lessen the trade deficit (currently about $600 billion/yr), but you have to tread cautiously here to avoid pissing off countries that buy our stuff. A lot could be done to keep U.S. companies from outsourcing so much... that would probably erase the trade deficit right there while boosting domestic GDP and well as gov't revenues (more jobs).
7. Legalizing drugs, as others mentioned, would have positive impact throughout the economy -- new taxes, decreased spending on interdiction, decreased spending on incarceration, decrease in crime overall throughout the country). Not an economy-saver, but a great weight off the nation's shoulders. Such a common sense win-win that only a straightjacketed conservative could resist. However, I'm sure that all DP Libertarians are in favor of it . . . . . .