Regarding Janice's comments on organization, I COULDN'T AGREE MORE! Preparation is key to feeling comfortable and successful in teaching the investigations. I have read a lot of math in bed this year, and find I am sometimes getting great ideas just as I drift off to sleep! It has also helps to have kids get the manipulatives ready. (For one lesson where each pair of students needed about 70 tiles, we got out the balance scales and only counted the tiles once. Then as each pair weighed tiles to get the right amount, other students plaayed Close to 100. Yes, it took more time, but we needed practice with the balance scales for an upcoming science assessment.)
I think as we become more comfortable with teaching/facilitating Investigations, my prep time will decrease.