We raised a Boxer for our own family, and later a Great Dane, and then another Great Dane, training them as guard dogs for other people.
There are a very few, simple rules we followed. First, the dog is a member of the family, and as such
never gets yelled at or hit. If he needs discipline, banishing him to the hallway was always enough to make him whimper and crouch down in complete dejection. The sooner this is over, the better: call him back in, tell him to be a good dog now, and rub his head so he knows he's OK again. Repeat the offense? Not on his life he wouldn't.
Second rule: the dog is a living creature of considerable intelligence, not a machine to start up when you're ready for him. He requires constant affection or affirmation, and a lot of exercise. Stop watching TV or reading that book for a few minutes, look over and say, get over here, you

and play with him for a bit. Let him climb up on the couch next to you. Get outside with him and run around. Whoever invented the frisbee is the dog's best friend.
Third rule: in public, the dog must be under total, immediate control. That means a leash, and it also means a shortened leash approaching people, even if he seemed to be ignoring them. Reinforce the training: dogs don't bite until I tell them to.
Basic rule: if you don't love them like a child, don't keep them. Dogs are too much effort to raise and train and feed and attend to without more of a payback than a security assist. If that's all you need, buy a gun and take it to the range now and again.