One area that tends to be the weakest for them is the rebuttal process. They anticipate arguments and prepare to refute them, but the skill of thinking on the fly is one that takes some time to build. How many of us, for example, have thought of the best response to someone's argument, well after the fact?
In order to prep them a little for the process, I come up with light topics and spend five-ten minutes at the first of each class working on building arguments and rebuttals.
First, I project a statement on the screen and ask them to quickly decide if they agree or disagree. Then, they need to jot down two reasons to support their stance.
Next, I ask for a volunteer who agreed with the statement to explain two reasons why s/he supports it. Before they do so, I tell students to listen to the volunteer's reasoning, and to try to write at least one rebuttal for the argument.
Once the volunteer has presented an argument, students offer counter-arguments. At that point, I just let them go, trading points and rebuttals. Because the topics are light and ones most can relate to, they are usually able to offer a response and we end up with a rapid-fire mini-debate.My hope is that after doing this a few times, they will feel more confident with thinking up rebuttals during our debates, when the topics are much more in-depth. I'll just have to wait and see.
You can find more ideas for debate here.