Having just constructed a worksheet full of wonderful problems is not a sufficient condition for a successful section. How you as the teacher use that worksheet is crucial.
The UGA's
Getting the worksheet to your undergraduate assistants sometime before the section starts is pretty important. The UGA’s are strong students, but seeing a problem for the first time when a student asks them a question about it in section puts them on the spot unfairly. Even having 10 minutes before section for the UGA’s to look a the new worksheet is better than not giving them any advance preview.
Pacing and Closure
You need to be alert to a situation where a group or groups is stuck on a problem and not progressing. A good habit here is for you to impose some “pacing”. Tell your students “Work on this problem for ten minutes and then we’ll discuss it.” After ten minutes have a possibly brief whole-class discussion of the problem. If necessary, you can give a solution or some strong hints. Then move on to the next problem on the worksheet. This provides a useful kind of closure, and prevents groups from getting bogged down. If some students are still uncertain about the problem, suggest that they work on it at home, and if necessary come to office hours.
Finishing Worksheets
Worksheets should be long enough so that, most of the time, most of the students will not be able to finish them during the section. But worksheets should be short enough that students will have time at home before the next section to finish the problems. The worksheets are something in between an assignment that must be completed in class and an open-ended resource of problems. In constructing them you should choose the number and type of problems carefully, and then make it clear to students that they should work all of these problems, either in class or on their own. (PDP TA Reference Handbook, 8-23-96)