Steve, over at Vodkapundit, has added another to his shoebox full of proposed amendments, which got me thinking that maybe this:
A constitutional amendment limiting government expenditures to a fixed percentage of the previous years GDP. If the government wants more money, they better do whatever boosts the GDP this year, or forgo the wish list for next year.
This could have the effect of turning socialists into capitalists, since you can't get the monies you need to subsidize poverty unless you first enact policies that promote prosperity. It also eliminated the need to demand that old law be repealed before new law can be passed, as repealing onerous regulation is the quickest route to getting the goose laying those eggs again.
Note that there's no provision for borrowing here. I'd be tempted to add some kind of exception for borrowing in the case of a declared war, but I suspect this would lead to a perpetual case of "Northamerica has always been at war with Eastaisa" or something to that effect.