I think generally empires simply push the envelope until they get themselves into a war they can't win, and they are overrun. It is possible that a war will put the empire under such strain that there is a revolution (see the Second German Reich and the Russian Empire, which both unravelled during—and because of—WWI). A more fortunate possibility is that when the rot sets in, a new leader will carry out a controlled demolition (see Gorbachev).

For the US, the best one can hope for is a President who becomes a reformist after assuming office (as in the case of Gorby, if it were obvious that they set out to reform the system, they wouldn't be allowed to assume power) and then one can only hope that when internal conflict is unleashed it doesn't spill over.

Otherwise the US is going to have to lose WWIII, or something.

Bush is a symptom, not the disease.