I just didn't reward till it was correct. To help get those sits straight I used the Do-whacka- stick. I love that stick, my heeling stick. Do it right or I'll whack you. LOL, love telling people that so I can see the look on their faces.
You know I'm kidding.
I learned it in a long, long ago obedience class. Put the stick on the floor, sit beside it. With a visual and physical help in the position they soon learned the position without the stick. A tape line or long heavy cord on the floor can help with heeling. We started heeling along the wall. I used the stick for drop on recall as well, sit at a distance too. Start them dropping or sitting at the stick, leave the stick, you move back. Pretty soon they understood the distance part then I removed the stick.
And I learned to position my treat well to the left so the dog anticipated it coming at her/him in a line which had them in a straight heel.