At my high school there was this incredibly overstated custom of holding doors for people. I know this probably sounds ridiculous if you are reading this and you did not go to my high school, but I am telling you-- it was a really a bit absurd at times. Part of this had to do with the fact that the campus was a collection of a whole bunch of small buildings rather than one big building so as a result, in between classes there was always a rush of people entering and exiting buildings walking down covered walkways that connected the buildings. People would hold the door for an extremely long time, even if the person coming down the walkway was 50 feet away or more. Our headmaster even made a comment about this little custom one time saying it was evidence of the strong sense of community at our school (questionable at best).
I remember specifically this one time that I was in a hurry and I did not hold the door for some people behind me (they were literally 20 feet away, still walking down the stairs) and as I was rushing away from the door (now 30 feet or so from the door) some kid yelled at me for not holding the door for the girl that had been "behind me." At the time I was very embarrassed for being impolite, although later I put two and two together and realized that the guy who had yelled at me was just trying to impress the girl he was with that day.
I would not claim to be any expert on manners. I have had one or two people tell me I do not have "social graces" (these people usually just suck though). Don't get me wrong-- I am a clutz, I endorse burping, and farting is cool with me if no ladies are around. People even accuse me of making an "I hate you" face and intimidating people on a regular basis. All that being said-- I think too many people just don't care about being pleasant and being nice any more. Don't get me wrong-- I know you're in a hurry, and I know the person you are holding the door for may not know you or may even suck-- but so what.
My dad always had this little sign on his desk that said, "What makes people different? Habits and Assumptions." My point about manners is that I think a lot of people just assume it does not make any difference any more if you are rude or polite- and more to the point-- they assume that being thoughtful won't be appreciated by anyone. In terms of the habit prong of the maxim, I think that despite my strange past with holding the door-- I do it out of habit most of the time now. Sure, there are times when I am in a hurry or I am having a bad day and I forget, but by and large I hold the door as a habit. I barely even think about it most of the time now.
My theory is that a lot of these little things like saying please and thank you, holding the door, etc., if you get in the habit of doing it, the marginal cost of doing it will eventually be close to zero-- chances are you will probably stop even making the conscious decision to do it and instead you'll just do it automatically.