When I talk to bands about how to motivate their street teams, I usually ask what they're doing to meet the needs of street team members. It's got to go beyond just asking people to hand things out for you. You've got to find out why they're interested in helping you in the first place, and provide as much of that benefit as possible as a reward.
As this article suggests, you should be helping all of your listeners "kick ass." When you get extremely involved in getting your core fans' needs met, they become ambassadors for you in just about everything they do.