For me personally, call waiting is a good feature to have on our phone because of our children - EVEN IF THEY ARE GROWN UP!
Sometimes their calls are of an urgent nature, so I have to click over and see, but just for a second. When I click over to answer their calls, I tell them I will call them right back, then tell the party I'm talking to that I have to go and call one of my kids, and I'll get back with them later.
One thing I try to do is respect a caller's $$$ being spent when they call me long distance. There is nothing more insulting and rude than for me to call someone long distance only to have them keep "clicking over" to the other caller during our conversation --- ON MY DIME, nonetheless!
So, when someone calls me long distance, I will ignore the call waiting UNLESS I know beforehand that I'm waiting on an urgent call. If I notice on my caller ID that it is one of my adult kids calling, I tell the person I am talking to that I will call them back rather than have them pay for the constant interruption of my call waiting feature beeping during our conversation. They respect me for not wasting their hard earned cash.
So, a courteous thing to do would be to remember "who" is paying for the call (if it is a long distance call) before you "click over" when the call waiting beeps for you. It is NOT a nice thing to click over to answer your call waitings, and especially click over and talk like you don't have a long distance party waiting for your return WHEN THEY ARE THE ONES PAYING FOR THE CALL in the first place.
In other words: BE POLITE AND COURTEOUS! RESPECT YOUR CALLERS TIME AND MONEY WHEN THEY TAKE THE TIME AND SPEND THE MONEY TO CALL YOU!