To encourage students to speak only in English in the classroom, make sure they know simple transactional phrases that they can use to manage their interaction. Often these phrases come so naturally in the native language that it's difficult to break the habit. To help everyone learn and remember this, make a list of these phrases on the board (or put them on a poster for the classroom) and add additional similar phrases as the need arises.
To help you get started, we have prepared three such lists on reference cards, which you can download, print out, photocopy, and distribute to the students. (You will need Adobe® Acrobat® Reader® to view these files. If you don't already have the software, you can download it free at www.adobe.com.)