Here's some old pictures of Winston about 5-6 months ago! He was only 4 months old here.
We trained Winston to sit by offering him a treat and pushing his butt down. He looked at us as if we were crazy and would walk around in circles. Then, he discovered his tail and would start to chew on it. Funny dog. After a few days of offering him treats and encouraging him to put his butt to the floor before we gave it to him, he finally made the connection.... we think.
Sometimes he just sits knowing we like it, but only to get back up again really quickly and start jumping towards the treat. And sometimes he sits when we tell him to sit with no treat in hand. Whether he really understands the command to sit is still up for debate. He sits on our command, but doesn't always understand other people when they tell him to sit.
But he has linked sitting to being a good dog. Look at him now. He sits even before we tell him to. This is how he begs for treats and affection, with his sad little puppy dog face.
Here he is outside sitting, hoping for a treat. First he's up, then he's down.