At the end of the fiscal year, every military unit I've ever been in tends to "use it or lose it" on their annual budget. Right around 15 September, we get a message saying that we had better spend thousands or even millions of dollars of this years' money, or else next year our budget will likely be cut by the amount we didn't use.
Military units (and DoD Budgets) should be rewarded for budgetary savings by receiving a guarantee that their budget will not be cut if they manage to SAVE money.
This is a cultural thing, and would HAVE to come from the top. Financial managers and Budget Officers would have to be educated on the new policy to break the expectation left over from "use it or lose it."
If every Division in the Army had each company, battalion, and brigade come under-budget by $20,000 (not too hard), this would save the Army alone $9.6 Million each year (some years it might not if all the budget was spent). Multiply this by several services, and other agencies within DoD, and it adds up to tens or hundreds of millions of dollars.
Of course, if the services NEED the money, they have it in their budget, but if they DON'T need the money, then they don't waste it.
Right now, the way the budget system works in DoD can easily be considered fraud, waste, and abuse (check out any Division's financial transactions between 1-30 September, and you'll see what I mean). Things need to change, and the change could be very simple.