There's sit... and then there's sit
It seems to me a lot of people are trying to teach their dogs a lot of behaviors all at one time. In particular they don't seem to get the heel and sit the way they want because the dog doesn't remain sitting. And the discussions seem to center upon what the handler is doing wrong to make the dog get up prematurely.
My question is... why are you working on heel, when your dog isn't solid on sit?
When I tell my dog SIT, I can walk away from it... back to it... talk to it... praise it... go to the end of the leash... come back to it and pat it... walk around behind it... walk away again...eat a brownie, pick my teeth, clear my throat, hum a tune... whatever, until I either resume the heel position and tell it to "HEEL" or tell it to "HERE"... or give it a release command. (And... you know, I hope, that you don't work on "here" until the sit is absolutely solid. Right?) A dog who can sit in a relaxed manner and remain sitting is one who can pay attention to you and the other things you want to teach it.
IMO, classic "obedience classes" rush this process terribly and are often counterproductive as a result.