Fortunately, there are some who are showing how it can work, including Barack Obama. This can be seen in concessions made by China after Barack Obama left. Andrew Sullivan (commenting on reporting done by Jim Fallows) helps to show that. On three of the four major issues that most people have been pushing Obama to push China on (the fourth being human rights), China shows progress after the President went home. That suggests (though does not prove; I don't want to be accused of post hoc/propter hoc issues) that the President had some effect while he was there.
And this is the essence of diplomacy. Loudly demanding things from countries gets you nowhere, even if you are the unipower. Perhaps, actually, especially if you are the unipower.