Say hello
Our Binx is 6 months old and we started obedience training early (10 weeks).
One of the first things our trainer taught us (she grew up with Boxers and knew how jumpy they are) was the "Say Hello" command. When someone is approaching you immediately put the dog in a sit and tell them to say hello. You tell the person that the dog cannot be petted unless he is sitting. (Then try not to laugh as the boxer tries to sit steady on a wigglin' bum!)
Easier to train the dog, I know, than the people that approach especially kids. I also take Binx to my childrens' school whenever I can at drop off and pick up so that he is around a lot of children and can practice the sit, stay when the kids want to pet him.
Another trick I thought was clever...you know how some people like to have a dog jump up on it's hind legs and put their paws on their shoulders by kind of slapping their own legs or their chest? She taught us to make that action another hand signal for "sit" so that when someone does that the dog's bum immediately hits the floor. Still working on that, mostly trying to teach my DAD not to do that!!
Anyhow, all it takes is practice, practice, practice, huh?!!