Always walk around with some scrap paper in your hand. You will not remember everything that a student may say. At least with a piece of paper and a pen can you start to note down some language which you can provide feedback for at the end of a student-to-student speaking exercise. Just take a step back and see how students are getting on, and don't just write down the language which needs correcting! You can also feedback to students when they produced some good language also!
Always include a feedback session and perhaps incorporate it into a game such as grammar auction. What do you like to do with student feedback at the end of the production stage?