Some of these are just my way of doing things but they are worth considering: YMMV.
1. Right at the start, emphasize very clearly that they must read the primary text closely, and discourage them from reading too much secondary literature. In most courses, reading around is practically compulsory; for GV201, this will almost certainly confuse them. At first, they should just read the primary text and maybe a simple introduction (Past Masters or similar). Anything else is probably a mistake.
2. Teach them in class how to follow a sustained argument. This will encourage them to do the same in essays. Point out when people are going off-topic. (Of course, you shouldn't discourage people from having their own ideas. But when you've started one discussion, follow it through.)
3. Make sure they understand the texts. I think it is good practice to spend at least one lesson per primary text, solely on analysing the text itself (rather than on discussing "issues raised").
4. When you do talk about more general "issues", introduce them with specific modern examples, to awaken their interest. But then try to bring the discussion round to the issue involved, rather than details of the specific example.