With small classrooms, too many children per classroom and not much time to do a class, it's quite inappropriate in my case to rearrange tables and chairs. Thus, what I do is to ask children to work in pairs whenever an activity is suitable for it. For children not to work with the same partner all the time, first they work with the partner on their left, then on their right, their front, their back, and so on.
Sometimes though, I put into practice some tips I get from 'cooperative learning' (Johnson and Johnson). I think it's quite interesting, for it provides detailed data to only arrange classrooms once at the beginning of the class in order to be able to work in big groups, small groups and pair group all along the class without moving the furniture again.