As well-armed as the military is right now, the only kind of revolution in this country that could succeed would be a coup by the military. But nobody stays powerful forever. There will come a time, maybe in a few hundred years, when the arms gap and information gap between the government and the governed is back to the point it was at during the Revolution, a new revolution by the citizenry may be possible, but an external invasion (or multiple external invasions) at that point would be more likely.
So what would it take? Beats me. Any kind of group would have a hell of a time forming any kind of conspiracy without the US government taking notice. The more people involved, the harder it is to maintain a secret, the easier it is for spies to infiltrate the organization, the less likely any revolution will be initiated.
Starvation would most likely work. Famines, rampant unemployment, and destitution strengthen the resolves of many. When the majority of the people are hungry and miserable, it'd be easier to rally them together for Revolution. We live in a country where 2/3 of the population are obese, so I don't think we're close to that point, yet.