When all else fails for me, I grab a picture book, take a seat on the teacher chair at the rug and whisper "If you can hear me, come join me on the rug. Let's tiptoe quiet as mice."
When the kids see the others getting up to go to the rug, they are usually quick to close their mouths and join us.
Other times I'll just start reading the book right there.
As this is my all else fails method - it usually means I'm pretty p.o.'d. So, I always follow up with the frustration talk. Usually they are very understanding and better listeners for a few weeks!
Other ideas: Teach your class a poem - post it and start to recite it when they're chatty, they'll join in.
Skip count - by 2s, 3s, 5s, etc. It's a great way to get them used to skip counting too! And by the time some of the talkative students join in they're getting practice in the higher numbers - which is what we all need!
Wait, don't ever talk over them. You are important and deserve to be heard!