Set up an ongoing, weekly campaign either with your class or the whole school. This may be a raffle using spent Star Cards, or anything else your creative kids can come up with. It may run for a few weeks, a whole term or a semester! Promote it at assemblies, in the school newsletter or class email.
Tell us what your weekly promotion was and how you promoted it. How did the kids react to it? How did it influence their travel to school?