Classroom routine, more than anything else I can think of, is the key to good classroom management. When the students are met at the door and they enter the room with the expectation that there will be an agenda and it will be adhered to, then they will meet the challenge. This is especially true in PBL classrooms where there is, not only, a teacher's agenda but each group may have their own expectations and agenda.
To help establish classroom routines, there are certain parts of the PBL process that should be present all of the time and will help students understand what learning is expected. This starts with the Driving Question (DQ). Once the Entry Event has been delivered the DQ should be posted prominently in the classroom and, I would add, on other materials given to the students as resources for the project. Within the first few days teachers should, purposefully, direct students to the DQ and remind them that this question is the reason for their learning.
Once the Entry Event and DQ have been delivered to the students it is customary to create a list of items that need to be understood for successful completion of the project. In most classrooms these items are called the Need to Knows or NTK's. This list of NTK's becomes one more thing that should be boldly displayed within the classroom. And, a good habit is to start class -every day- with a discussion of where the students are as far as learning about these key components of the project.
In New Tech classrooms we not only have a list of NTK's but we make a list of things that we Know. The combined list is our Knows and Need to Knows (K/NTK"s). When students are able to list their "Knows" they are reinforcing the idea that they come to the project with certain skills that will be needed for successful completion. In revisiting the lists each day we are able to line out NTK's and add them to the Knows section. Leaving the NTK's up but just lining them out helps students visualize their learning.
The final key component to successful PBL classroom management is well constructed rubrics. Rubrics let students know what they are being assessed on within the project. And, with set levels of below, on, and above proficient (as a minimum) the students know day to day how much work needs to be accomplished. A fellow teacher adds a special category for students who have completed all line items on the rubric - The Wall of Awesome. When students do everything required and do it well but want to go further then they are put upon the wall.
And so a teacher who has a set daily agenda, posts and revisits the DQ as well as the K/NTK's, and requires groups to have their own agenda, will have a classroom that glows with learning. At no point should a students ask "what should we do now?" Wouldn't it be great if everyone of your 32 students had their names on your Wall of Awesome?