List the techniques that you want your students to use in their persuasive speaking/writing exercise. These may be employing repetition, using comparison/contrast, furnishing proof, demonstrating consistency, providing solutions, creating narratives. Briefly tell them how you want them to speak if they are to "sell" an idea. Ask them to be ready to take notes, to agree or disagree with the speaker.
Give the class a topic. It should be something that gets them all involved, such as convincing parents to let them have a pet, or the principal to let them have more extensive sports facilities. Offer an incentive: the best letter will be delivered.
Have a brainstorming session: What persuasive arguments or techniques do they need to use to convince the reader of their point-of-view? Ask them for ideas; post the feedback in point form on the white board.
Giving examples or references will be considered proof of persuasion and will strengthen their cause. Encourage them to give concrete examples. The more involved they get in the speaking part of the exercise, the better their performance will be when they start the writing part of it.
After some ten points have been listed on the board, ask them to organize an essay or a letter citing those points as references and making a first draft. Instruct them on how to use the points. Advise them to mind the tone, expression, spelling and grammar, and write with clarity and confidence to win the prize.