Well, America's in relative decline in that it's economy is slowly growing but many other countries like China, Brazil and India etc are fast catching up and China's likely to overtake in a few decades.
So in some way's America's decline is going to be like Britain's. Living standards will continue to increase, people are still going to work in relative safety and Hollywood will still make it's blockbusters. The main thing that America will have to mentally adjust to is not being the worlds sole-superpower. America won't have unquestioned military superiority of China continues to ramp up military spending, America won't be the first place where the worlds foremost intellectuals will go nor will America be at the very forefront of scientific research.
But given it's large population America won't shrink into the middle-power league like Britain has. America will be either joint superpower or second superpower to China for most of this century. I'd be surprised if Americas's not in the top 3 world powers by the end of this century. Assuming some sort of disaster, political, civil or environmental, devastates America.
In all likelihood, America will in the next few decades have it's own "Suez Crisis".[footnote] In British history, the Suez Crisis, occurring in 1952 i think, is when Britain and France conspired to create a war between Isreal and Egypt that would allow the UK and France to step in as "peacekeepers" to re-take control of the Suez Canal which had been nationaised by Egypt. The USA disapproved of this and threatened to call in the debt that Britain owed the US, forcing an embarrassed Franco-British withdraw. The point of this is that it brought to everyone's attention that Britain was no longer a major power.[/footnote]Perhaps the USA will perform a military intervention in South America one day and China, disapproving of this, threatens to call in it's debts and America's forced to withdraw. Or maybe it will be something more symbolic like China beating the US into putting a man on Mars. It may be something like that, a few political embarrassments for America, that will lead mean the country will have to come to terms with no longer being the sole superpower of the world.
Still, i don't think anything drastic will happen, and America will continue to be a major world power, just not the sole world superpower.