Mark Thoma has a cool chart about where your tax dollars go. When people want to fix the federal budget, they generally cite foreign aid or pork barrel spending or government wages or corporate welfare to cut. But these are all very small parts of the budget (they are in the "other" part of the graph...a total of 4%).
Suppose you're a congressman. You want to cut the budget? You might want to start with Medicare, Medicaid, or CHIPS which are health insurance for the elderly, poor, and children respectively. That will upset a lot of your constituents and I'm sure you're hear about it in the next election when your opponent tells the district you hate some of America's most vulnerable citizens. OK so you can cut Social Security instead, but that will only upset a smaller group of people with even greater fervor. Maybe you could cut the safety net expenses, but that won't be politically smart during a recession. Military's high, but a lot of this is concentrated in major contracts and military bases: a mighty large group of people to upset and has the public sympathy of defending the nation.
In other words, there's no easy way to cut the budget. The reason why these are the big areas of the budget is because they concern people with a lot of public sympathy, sympathy they were able to transform into payments. And it's that sympathy which secure those payments now. If they didn't have that public support (such as from Reagan's famed stories of "welfare queens" which promoted welfare reform), then things would be different. The best way to cut the budget is if hundreds of thousands of elderly people were using their social security checks to fund terrorism.