We spend $522 billion per year on defense (http://www.borgenproject.org/Defense_Spending.html). China, our biggest threat, spends $63 billion. Who the hell are we protecting ourselves from? Chop that down to $200 billion per year so the hawks don't freak out, and you've got $322 billion right there.
We have almost 2.5 million Americans in prison (http://www.csmonitor.com/2008/1020/p08s01-comv.html). It costs on average $24,000 per year per inmate. That's $60 billion. Cut that 1/3 by using alternative sentencing and treatment for dug offenders, and there's $20 billion.
So in about 10 minutes I've found $340 billion to be used toward our deficit. Of course, Wall Street and the banking system are eating up twice that much just to cover up their mistakes, so maybe it's a losing battle. But there's a start anyway.