In my class when I needed the students attention, I would say, "One, Two" The students reply, "Eyes on you" Then I say, "three, four" and students reply, "talk no more" This worked well and the students enjoyed it. Sometimes I would have a student reply "shut the door" they would quickly fix their mistake. However I noticed that you wanted a quiet signal, in which case the give me five is a great idea. Sometimes the kids would get loud with out chant. Good luck!