How can I teach a grown dog to sit?

Well, I came to update the status on this. So basically, I succed to make him sit with his favorite toy, though, if I don't have it, he simply ignores me completly. About making him walk on the leach, well, that seems to be impossible. No matter how much times I pull him back and tell him "slowly", he'll keep on pushing me. I'm running out of ideas on what to do... Maybe I'll just give up and let it be. He's too used to be like that and I doubt we can do something to change his mind now. Maybe a professional could, that's a option to consider.

Thanks for every tips you gave me though.