A student-centered classroom is one in which the majority of the learning is actively done by the students. This is in contrast to the more commonly though of set-up with straight rows facing a lectern and students listening to teacher-presented material the majority of the time. Student-centered, however, doesn't mean that the teacher is a highly-paid babysitter, having no purpose other than as a warm body in the room. The teacher's role changes throughout the year. Teacher/student responsibilities and physical environment are areas to consider.
In the beginning the teacher is more of a presenter as the rules and procedures are introduced. Since a student-centered classroom features student-choice and groups of different sizes working on potentially different areas, classroom management is a critical feature to have implemented. Procedures must be set, preferably modeled and created together with the students, for teacher expectations when students are working alone, in a pair, or in a small group; when students are transitioning from one area or activity to the next; or when someone or a group has a question. It is really helpful to the students, once a classroom has been operating in this manner for a year or two, to videotape an example of what works and show it to the new classes. Lead them to define what they observed that was beneficial to learning, then post it in a prominent place around the room. This makes it the students' responsibility to refer to the guidelines for various situations as they arise. As the year goes on, the teacher's role changes. Although there will always be time when direct instruction of the whole class will have a place, the teacher in a student-centered classroom becomes that of a facilitator rather thatn a presenter. This means that the teacher makes it possible for students to acquire their own knowledge rather than the teacher giving it to them. Teachers in this type of classroom will manage the class by constantly evaluating the students - this is not the time for a teacher to sit behind a desk and catch-up on grading! Rather than give answers to questions, the teacher guides the students to acquire and synthesize information on their own or with other students.
Admittedly, sometimes a teacher has little control of the physical set-up due to funding or structural concerns, but flexibility is key. If students are to work in ever-changing groups - both sizes and make-ups - then they need areas in which to do so. Creating clusters out of desks or using tables in lieu of desks could be an option. Small rugs places around the edges of the room with clipboards easily accessible gives students another option for a work area. Access to materials needed, whether computers or construction paper, needs to be student-friendly to limit the number of times a student has to ask the teacher for something. Plastic see-through organizers that are clearly labeled organize materials and look attractive at the same time. A variety of materials is needed. Parents can be invaluable when it comes to this need. If you have, for instance, four sections of one class, send a letter home to the parents of each class asking for donations of different materials. One class could bring in old magazines, another one posterboard, another a package of adhesive notes, etc. Write grants for the pricier things you need. Donorschoose.org is a fantastic source for teacher funding.