Group learning: using students to help each other

Twice this past week, I had students break into small groups to work through problems. This strategy worked very well, and I plan to use it periodically in the future.
On Tuesday, we had a class period devoted to reviewing for the upcoming test. The previous week, I'd also taught a review, and it seemed like a drag for both me and the students. Part of this was the fact that the review was scheduled for 2 periods, but I think the bigger problem was that I was just having students work on problems one at a time and then take turns showing them on the board. My students love board work, but doing problem after problem for 90 minutes, especially when none of the material was new, got boring. So, for this week's review, I broke the class into small groups (strategically mixing up ability levels and behavior types) to work on the problems. Even though it meant that there was a little more chatter than there otherwise would have been, I think the group work was a great idea. All the groups were being productive, and the students stayed more engaged than they would have if I had just done the board routine. Another cool thing about having students work in groups is that I was able to get the chatty students to focus just by my physical presence in a way that doesn't seem to work as well in lecture. I moved between groups pretty frequenty, and if I just looked over or stood next to a group that I noticed was getting off topic, they often noticed and kept working. This was a particularly important discovery for me, since I've admittedly been having a hard time calling out kids who talk about non-math topics (side note: I still need to get tougher about that; group work isn't an escape hatch).
Today, I broke students into groups again. It didn't go quite as well; students were singing and keeping beats, and I didn't crack down on them hard enough. Nevertheless, I think today's group work was still beneficial. Last period yesterday, I had covered a topic that students were struggling with (graphing 2-variable inequalities), and I was given 2 periods today to cover a topic that I thought could be addressed in just one lesson, so my second-years gave me the OK to take first period to do some more work on graphing 2-variable inequalities. I had the students do some graphing warm-up problems, then went through a 2-variable inequality problem with them, and then broke them into groups. To me, the coolest thing that came out of it was seeing how students were able to help each other. One girl was struggling, and no matter how I tried to explain it to her, didn't seem to get it. Another girl in her group gave her lots of help, and she seemed to be understanding more by the end of the lesson. Side note: a really cool thing happened with the same student during second period. I asked her if she could try a problem on the board, and she said she didn't think she could do it, and I said, "I'll help you do it on the board. We'll work on it together." She immediately perked up and said, "OK!" She went to the board, did the problem out, and got it right! And pretty much all I did was stand there and tell her she was on the right track once or twice when she asked me. All she needed was to feel like she wasn't completely on her own. Neat.
The next time I do groupwork, I'm going to lay out some rules before I divide the students up. Talk to only the other students in your group, and talk only about math. Stay in your seat unless Ms. Long Skirt Blue Jacket gives you permission to get up. I'm also considering have students later hand in the problems they did during group work just to make sure everyone stays on task.