Do you find that certain groups of students always sit together and start talking? When I was at school, one of our teachers had a great method. She stood at the door of the room and as each student entered, she directed them to a specific seat. She would put friends as far away from eachother as possible in order to avoid talking. Left to their own devices of course, they would always sit together.
She then followed the idea from a previous poster. She would simply stand completely still and completely silently at the front of the room, but somehow very assertively, just staring around the room, making eye contact wherever possible. The class would quiet down, she would quietly say "Thank you" and continue. Over time, most people simply entered the class and sat down quietly on their own.