My method is a little crazy, but it works well for me. The research on cooperative activities says to assign roles for each member. I find I end up with several individuals doing individual roles while they sit in a group.
My favorite method is to plan to blow the first activity. I give the assignment, explain that everyone in the group should work together, then turn the groups loose on the assignment. I moniter and record examples of positive cooperation in each group. I also record any negative cooperation.
Then we meet as a class to discuss how well each group worked together, what went well, and what didn't. I read my notes to the class. I usually don't target any specific group or individual in my comments. I especially mention phrases I heard that I would like to hear from each group, such as "good idea", "cool!", "awesome", "what's your idea?", etc. Then, as a group, we make a list of helpful things to say to each other in a group, what we shouldn't do in a group, and what we should do.
Now and then I'll have a student who needs more guidance in group work, but mostly, after this first lesson, I have more positive results than by following traditional methods. I meet 5 minutes after each lesson to discuss what I've observed in how each group worked together. Normally, I find groups far exceed my expectations.