1. Any more than four students is less effective. With five or more students there is often a student or two who has a tendency to not contribute. In a smaller group there is more peer pressure to contribute.
2. You will find that more advanced students will more readily work together in English. Beginners and low-level students will have a much greater tendency to speak in L1. So you should adjust the tasks accordingly. Low-level students may need more modeling, more scaffolding.
3. Make sure the instructions are very clear and do a demonstration if necessary. If there is some confusion about the activity, students might not ask for clarification and not do the activity.
4. Mix up the people in the groups (or pairs). Do not let the people who are sitting together be in the same group. These people who sit together are usually buddies who are sometimes “too” comfortable with each other and often too lazy. If I have 20 students and want to make groups of four, I will start at the front of the class and go up and down the rows of students counting off the students to five: 1-2-3-4-5-1-2-3-4-5-1-2…etc. Then I tell all the “ones” to get together, the “twos” to get together, etc. This effectively causes a mix of students who are not “too” comfortable with each other. You will find that such a group will perform better and put more effort into the exercise.
5. To generate more English, ask the students to record their role play or activity. Almost all of our students have mobile phones that can record, certainly one of the students in a group has one. If you ask them to make and record their product, their role play, they will focus on the English earlier in the activity. They will often work out the sentences in English that they want to say, discussing the grammar and vocabulary, sometimes even working out a script.
6. Alternative #1 to the suggestion above, give every student two peanuts in the shell or some other cheap wrapped-up treat. While they are working together on their activity, if someone speaks in the L1 they must surrender one peanut to the person who caught them. Even adult students have great fun with this and it keeps everyone on their toes to really try to keep everything in English.
7. Alternative #2 to the suggestions above, appoint one or two “English policemen”. Their job is to patrol the classroom and catch and remind students to speak only English. Of course, I’m sure you also circulate around the classroom to do this but sometimes there are just too many students.
8. If the students know or think they will have to perform their activity (or play their recording) in front of the class, they will put more effort into it. You may not have time to have every group or pair perform. Choose some “Winners” and “Losers”. Take two thin slips of paper, maybe about 1×4 cm. On one write “Winner” and on the other “Loser”. After the students have had some time to do their activity, tell them you are going to pick a team to come to the front of the class to do their role play or activity for everyone. Hold the papers in your hand so they cannot see what you have written. Go to Team #1 and ask them to choose one paper. If it is “Loser”, tell them you are very sorry but they will be unable to perform their role play for everyone. If it is “Winner”, call out “Congratulations!” and invite them to the front of the class to perform. It is so funny to see students rejoice when they “lose” and be so sad when they “win”. This technique makes all of the students participate seriously in the activity because they don’t know if their team will be chosen or not.
9. For the teams that perform, you could give them some sort of score or even have the audience of students give them some sort of rating. You could award them with more peanuts or some other token cheap prize or just a round of applause.
The main idea is to keep it fun but also set up nudges, choice architecture, or the dynamics that will prompt the students towards the behavior that you want them to follow.