I found this idea on Pinterest - one of the few I think really work for a secondary room - and latched on to it. The original post came from this Tumblr account. I found it too late to use for this past year, but I'm definitely using it this year.
Every conference I've ever been to (including when I heard Charlotte Danielson speak) and almost everything I've ever read has stated that one of the best methods of getting rid of the noise is to have a silent signal, like raising your hand. That system doesn't work for me, but I need something. This is the solution.
I am posting these letters on my white board in the front of the room, where they are clearly visible. Whenever it gets too loud, I take off a letter, starting with the "E".
Eventually, this is what's left...
And this "NO" means exactly what it says - NO more. The classroom basically goes on lockdown. No talking, no working with partners, no getting out of seats, etc.
I like this because it gives the students a chance to regulate themselves and seems a lot clearer than putting strikes on the board or what-have-you. Having this decoration up all the time shows that expectation is always there, as opposed to being able to "forget" when there are no strikes.