L.C.
My 5 year old son recently stopped sucking his thumb! I was constantly nagging him to take his thumb out of his mouth. His dentist explained to him why he had to stop sucking his thumb. But he continued to suck his thumb -- especially while sleeping. Finally, I bought a product called Mavala Stop. I ordered mine through Amazon. (Read the reviews. You will learn a lot.) It is nail polish that tastes bad and therefore acts as a reminder for my son to stop sucking his thumb. A tiny dab on each thumb nail every few days was all it took.
Every once in a while, my son backslides and starts sucking his thumb again. He actually asks me to put the "dots" on his thumbnails to remind him not to suck his thumb. My sons teeth are pretty well aligned again. I wish I would have tried this method sooner. Good luck! It's a very hard habit to break.