Thumb Sucker!

My son just turned 4 yrs old and will be starting school VERY soon! He still sucks his thumb, but mostly when he's SUPER tired! He's ruining his beautiful (baby) teeth. I would REALLY like him to stop the thumb sucking before he starts school and really before it's something he will latch onto forever. Sometimes I even catch my best friend sucking her thumb and she's in her early 30's!!!! Any and ALL suggestions, advice etc. I am open to ANY AND ALL!!