We were put on this Earth with the instrument of humming ingrained into our being. It is, therefore, quite acceptable to hum with very few exceptions.

First and foremost, one must never hum the Jeopardy theme song.

Secondly, one must always remember to keep the mouth closed. To do otherwise is to invite peanut throwing from passersby. As we all know, it is worth twelve points to score a peanut in someones gaping mouth. Though we shant talk of this here, it is important to note that on airplanes this is especially important and easy to accomplish. It also helped slow the snoring epidemic of the early 1960s in airplanes.

Finally, while humming, you must focus on the letter "m." To do otherwise is to slip into the nasal "n" sound, making you look and sound quite ridiculous to the clientele you are attempting to impress.