My son sucked his thumb until he was about to start kindergarten. Here's what worked:
1) He and I went online and we found a site that showed what teeth look like when you suck your thumb - not pretty. They also showed a pic of the metal device he would have to get if he couldn't quit himself. He and I agreed it was time. He also didn't like the idea of sucking his thumb in kindergarten with all the big kids.
2) We sent his blankie (thumb-sucking and lovies go hand-in-hand) to a little baby who doesn't have his own blankie. (Actually went to my parents in case but we never needed it again.) My son and I put the blankie in the envelope together. He got to pick out a little something from Toys R Us for being such a strong boy.
3) We created a "robot thumb." I wrapped duct tape around his wrist and thumb (with kleenex underneath so it didn't get sticky stuff on his skin). I was lucky bc my son thought this was cool and never had an issue with it. I took it off during the day when he wasn't sleeping (usually). But he didn't mind it. We kept this up for several weeks.
Come up with something that your daughter would think is cool. Like, it's a Hannah Montana microphone or something.
4) No more thumb sucking! Totally worked like a charm.
The key is to "sell it." Get her involved and make the whole thing sound like a big girl thing to do. If you force it, the result probably won't be positive.