Ted: I have tried some of the things you suggested like encouaging after classroom discussion group through telephone number exchange, sometimes holding conference presentation to get inter-group exchange, as well as tutor-pupil pairing. As you said, sometimes it works sometimes it does not depending on the chemistry inside the group.
Giving group project is the most enjoyable event. Once I gave the real world problem group project for them to come up with pricing proposal for the painting job of our classroom. Both groups work very hard to work out wonderful proposals. It is amazing to see how they split responsibilties: some work on finding the best price of paint in town, some do the measurement and some work out the area need to be painted. One of the students even find out from his dad that the cieling paint should be different from the wall paint. Everybody enjoyed and everyone learn a little bit about how mathematics can do in their live and how people can work together to make impossibility into wonderful result. However, as you can imagine, there was some students in the group still had very little idea about how his group mates coming up with the result, although he had overall picture and great contribution to the group by searching in news paper for best price.
However, you and Ben definitely have given me many good ideas, especially your wonderful rewarding system.