In one of my classes, the professor always starts class with the greeting "Good afternoon, scholars!" I'm told that he starts morning classes with "Good morning, scholars!" It's kind of cool that he has this thing trademarked. people know him for it. it takes all the ambiguity out of when the lecture is starting. It's enthusiastic.
I don't know if I ever started class the same way twice...I'd clear my throat or say "alright" or something else awkward like that...or maybe just stand up in front of the room until the talking quieted down. I played music before class and tried to time it so that when the music stopped, it was time for class to start.
I'm not looking to rip off a trademarked class starting phrase, but having something unique, effective, and memorable wouldn't be a bad thing (if I could develop one).