One of the first things we started working on when Baby W started OT was W sitting. He was young and it can be common and the OT said most kids grow out of it. However, we have a long history of continuing to W sit long after peers have stopped (ahem, its certainly possible I still did it into my twenties and only stopped after I first dislocated my knee and the W sit position would then throw the never-quite-the-same-knee out of place) so she wanted to work on it with him.
He will go long stretches of time without doing it but will periodically revert back to it. During his "off" times, I've corrected his seat so many times that I don't even have to physically move him anymore, I can just give him a reminder and he'll move himself into a better position :p He kept W sitting during his last OT session and his therapist asked what I normally say to him to keep him consistent. She was going to say what I normally say while moving his body into position. But as soon as I said out loud "W, sit on your bottom", he moved into the correct position without her help--she was really impressed that he followed directions so quickly and correctly :p Kid may be stubborn but he is resigned to the fact that sometimes, you just have to do what Mom says!