Attention
Attention is the sine qua non of all dog training. Only an attentive dog can learn, hear, or obey. So it is the first thing you must teach your dog.
Training various sorts of attention is not very difficult. The challenge for me is deciding exactly what you want--
One extreme is a perfectly attentive dog that is always focused on you. This is great for most things, but it requires constant focus from you. The dog tends to not be able to entertain itself and is somewhat annoying. Even if you go to the park it generally ignores other dogs and stands one inch from you waiting for you to throw the ball. It can also be very difficult in movie work to make a dog like this appear to be someone's dog because the dog is always staring off-screen at dad!
The other extreme is a very independent dog that does not need anyone and does not pay attention. These dogs are sometimes easier to live with, but are hard to train because they are constantly focused elsewhere.
Obviously the ideal is a delicate balance of these two extremes, but to me it is a challenge to decide precisely where is the optimal balance.
The most basic method of teaching attention is also probably the best. You take treats and hold them in each hand. You sit or stand in front of your dog. Your dog will sniff around your hands trying to get the treat. After a little while he will look up at you with an expression that says, "Hey, idiot, open your hand!" As soon as he looks into your eyes, give him a treat. Do this over and over again, varying which hand gives the treat. Do this with an 8 week old puppy! Or whatever age your dog is. Soon he will start to get the idea that the way to get the food is to look at your eyes. Then you start holding his gaze for a little longer before rewarding, then longer, and soon he is staring at you! At this point you go back and proof the exercise by having mild distractions in the room and still waiting for his attention before rewarding. You can also mix this up by using toys instead of treats.