You're probably already got some brand champions, and you don't even know it. They're probably not A-list actors or celebrities, or anything like that, because if they were, you'd definitely know it... but that's alright. Personally I think word-of-mouth can even be more effective than some kind of big-name endorsement, because I think people probably realize the celebrity is getting paid and doesn't actually drink that kind of orange juice, or whatever. An endorsement from a friend is more likely to get me to try a new restaurant, though.
So you've got fans, and they've got great things to say about you. How do you leverage them?
This article on openforum.com has some tips - first, you've got to find them. See who's talking about you, positive or negative, and connect with them. Continue those conversations. Create communities and spaces for them. Give them mountaintops to shout from, and they will.