There is actually a pretty well defined etiquette surrounding the US Flag and it's all pretty well written out in Order Code RL30243. The "rules" include many very specific provisions that fall under "respect" for the flag. One of these is that "the flag should not be dipped to any person or thing."
http://www.senate.gov/reference/resources/pdf/RL30243.pdf
There are a number of "rules" that many well meaning Americans break. For example, in efforts to demonstrate their patriotism, some Americans will put a cheap replica of the American flag in the window of their car and display it until it's tattered and dirty. This isn't good.
http://www.senate.gov/reference/resources/pdf/RL30243.pdf
They will also put a flag up outside their house on a wall pole and leave it there. While you can display a flag 24 hours, it should be illuminated at night, not be left out during inclement weather (unless it's an all-weather flag) and should be destroyed (ideally by burning it, ironically enough) and replaced when worn. Otherwise, put it up in the morning and take it down by dusk.
Another one commonly screwed up is when people use the flag for advertisements or as decoration. So, technically, patches on one's gi of the USA flag are a no-no and disrespectful.
There are many more, but ultimately, it's etiquette. While the rules are clear, it's been pretty well established that there is no enforcement.