Seems like everyone has it covered and I'm definitely not a trainer. I can only say that early on I taught my boys to turn their back and ignore the negative behavior, same thing with mouthing. When someone goes to pet and Rocky tries to mouth, we say no and pull our hand away. We then try it again and if he doesn't mouth we pet his head and tell him good boy, basically teaching him, if you want attention you will have to earn it by behaving. Probably 90% of the time he gets it. Greeting people at the door is something we are working on, getting him to sit and I inform company coming over what I would like them to do so that it is consistent and that Rocky knows expectations. I don't know about the clicker training concept but that might be a way to reinforce positive behavior. I know there is a lot of controversy over remote collar, we have one but rarely have to use it and when it is on him the level is turned so low that I put my own finger to it and you just feel the poke, can't say that it is any crueler than a choke or prong collar. It might be the quickest way for your GSD to understand the expectations of greeting or jumping on people and when it is reinforced, it will be engrained good vs bad behavior and you won't even need it anymore. You might want to read some articles on the "alpha dog" stuff, who's in charge, etc....GSD's are a special breed and understanding them better might be helpful.