Marbles in a Jar: This is probably one of my favorite motivating tools in a classroom - it's easy and kids of all ages love it. There are so many ways you can go about motivating students with this. For beginner students I usually drop into the jar one marble per German sentence spoken by a student. As they get more advanced, then perhaps I add a marble when they speak two sentences, a paragraph and so on. And after we've had a lesson on a particular grammar concept, I likewise reward them when they apply what the've learned. When the jar is full, they get a reward. The possibilities are many with marbles in a jar because you can adapt it to the needs of your students. The only disadvantage is if ever your jar falls to the ground, you may have lots of marbles to pick up. (Yes, it's happened to me before.)
Language Coupons: This is used a lot in language classes and it especially works well when students are more advanced. Basically students are given a certain amount of coupons, let's say twenty and whenever they don't speak German, they need to give a coupon back to the teacher. At the end of a predetermined time, whichever students have above a certain amount of coupons, they've earned their reward. Alternately, you can have the students be the "language police". If they hear one of their classmates not speak German, then they get a coupon from that student. I only go this route if the students are mature enough not to have their feelings "hurt" when another student gets their coupon.
Visual Desk Reminder: For this motivator students have a sticker put on the corner of their desk. If they speak English or whatever their mother tongue is, they give the sticker back to the teacher. If they are able to speak German for the whole alotted time, then they get to keep the sticker and choose a reward. I've heard also of teachers putting a piece of gum or some other treat as an incentive for students as well.
Charts: Though I haven't used charts very much, I've seen teachers use "race" charts with success. They divide the class into groups and have each group select a name for their team. Each team is placed at the start line of the chart and then advance according to the German speaking rules of the classroom. You can really be creative here - I've seen everything from simple car racetracks to "travelling around the world" on a real map.