How do I get my 3 year old daughter to stop sucking her thumb?

She was born w/cleft palate and was unable to "suck" on a pacifier or bottle until her surgery (at 1 year); she then stopped sucking her thumb because the surgeon required arm restraints for 3 weeks ... started sucking her thumb again 2 months later! Her pediatric dentist really wants her to stop before she turns 4. So do I! Any ideas?