First, I crop such images to remove part of the background. That alone makes the background somewhat less intrusive. But often I don’t want to eliminate it entirely; I merely want to de-emphasize it. One quick fix is first to blur the background slightly and then to adjust the saturation of both background and subject.
Lifehacker links to a great short article on how to get rid of the background clutter in a photo. The article from Photodoto suggests:
Go to the article – Quick fix for cluttered backgrounds for more.