Listen, and listen some more
Nice link, Romer! All really usefull info. I LOVE how the article says and repeats several times the concept of LISTENING(Hint, Hint)!
Listen first, then listen some more. Nothing can say "inconsiderate" more than some station breaking into an ongoing QSO. (Now in some cases, it can mean "newbie" if the breaking station immediately apologizes and then requests to use the repeater)
It is good form to leave a second or two at the end of a transmission as a pause before talking again so that any station wanting to break in has opportunities. If someone just starts calling during that pause, it is a dead giveaway that they weren't listening first. So be patient. Listen first and then break in if the repeater is busy. Everyone will think you know what you are doing.