In my intermediate French Immersion classroom I am constantly needing to try new systems to encourage students to speak in French with their peers. One of the most successful systems that I have implemented has been a 'passport system', where each student receives a new passport at the beginning of every month and they get a hole punched in their passport if they are caught speaking English during class time. The passports are collected at the end of the month and used for French marks and--potentially--for prizes, if the teacher so chooses... :)
This document is a pdf with printable passports for the 10 months of the school year: September through June. The passports are, of course, in French.