It may sound kind of silly when I type it out, but here's what I do.
Raise my hand and say (rather loudly), "On 5, you are getting quiet."
Then a little quieter, "On 4, everyone is heading back to his or her own seat."
A little quieter yet, "On 3, you are getting ready to listen."
Even quieter, "On 2, all eyes on me."
Then quietest of all, "On 1, . . . . (start giving instructions)"
Of course, I don't always say the same things, but the point is I raise my hand to get their attention and get a little quieter each time as I count down. Sometimes everyone is ready to listen by the time I get to 4!!!!!!
It works very well for me and my 4th and 5th graders.