nearly 15% of what the federal government spends is coming from borrowing it's very hard to actually figure out where one might come up with half a trillion dollars or so to close the gap.
The New York Times has come out with an online deficit-reduction form which allows you to pick and choose among dozens of measures designed to increase tax revenue or slash spending in order to get the federal government to stop spending more than it brings in. I suggest playing with it to see just what it's going to take to get budget balanced.
The funny thing to me is that this is really extremely simple math (TAXES - SPENDING = 0) however it seems to be somewhat more complicated in practice than in theory. This is one reason why I am a theorist.