First: If you want me to call you back, leave a message. Otherwise I assume that either you didn't consider the call important or it was only relevant at the moment so there is no need to call back. For example, someone calls because they are going to a movie in a few minutes and are inviting you to go. No point of leaving a message if you didn't answer because it would be obsolete in ten minutes. However, there are times when I don't follow my own principle. I rarely leave messages on my parents phone because I know that my mom almost always calls me back if she misses my call.
Second: Certain rules of the road should apply to pedestrian walkways. Generally people should walk on the right side of the sidewalk and pass on the left. And if you are walking with a group of people, the person on the edge must yield to oncoming pedestrians. Don't push someone off the curb or into the mud because you can't handle stepping behind your friends for two seconds.
Be careful when you stop in the middle of a walkway. People may be walking quickly behind you. (It really irritates me when I almost run into someone because all of a sudden they stop, but I am definitely guilty of doing stopping without warning)
Third: When you line a trash can, push the bag down before securing it to the bin so that there isn't a huge air bubble trapped under the trash bag. It just makes sense, because otherwise you throw away a paper towel and the garbage can looks full.
Fourth: Gum should be spit in the trash can, and if there is no trash can, then it needs to be spit in a bush or something, not where people are going to be walking. Nobody wants gum stuck to the bottom of their shoes. It really isn't that hard to find a trash can or bush.
That is all I can think of right now. Feel free to add to the list. I am curious about your maxims.