I'm also in the middle of a long-term assignment. The first thing I did was change the seating chart to separate the talkers. That was somewhat effective. Now I'm starting to send students to the office to put some more fear in them. Detention slips work sometimes, but there are some kids who get them weekly, so it doesn't work.
I'm subbing for 8th grade history till the end of December. I also went to talk to 7th grade history teacher who had all these kids last year. She told me they were the worst class in her whole teaching experience - that info helped a lot
. This teacher did tell me that she had to have a lot more structure for that group of kids, so I'm modifying my lesson plans and will be giving them very little independent work, since that's when things tend to get out of hand.
I'm a history major, so I enjoy the subject and try to make my lesson interesting and engaging. Sometimes kids just don't care about the subject I teach and they don't care if they are getting an "F" in the class. I can only motivate them so much, but they have to take responsibility for what they do.
I also do a lot of praying - God knows I need help!
Good luck! How long is your assignment?