Contrary to popular belief even and old dog can learn new tricks.
The number one rule in teaching any dog a trick is being sure you have its attention. Much like working with a young child, if you don't have their attention you aren't going to get very far.
The next step is positive and intermittent reinforcement. The average dog, young or old, can be taught to obey the command "sit" in just a few hours. This is then reinforced over the next couple of days. Rewarding the dog for correct behavior should be varied. For initial positive responses the dog is given a small piece of a treat. This should be alternated with positive attention specifically verbal praise and petting. As the trick is mastered it should be executed upon command by the dog. At this point in the training, you vary your response to include: praise, petting, treats and no recognition.
While it seems cruel to expect the dog to obey a command without acknowledgement, it is essential in a thoroughly trained dog that they execute a command even under conditions where acknowledgement isn't possible. For example: in an emergency situation such as a car accident in front of your home, it is necessary that the dog execute a sit and stay command and remain there so that you could attend to the situation at hand without worries that the animal was going to be underfoot. The same could be true for a spill in the kitchen or the need to discipline a child. There are any number of situations, like a visitor at the door, in which having the dog obey the command is key to keeping your sanity.
Once you and your pet have mastered the command for sit, you can move on to other commands. It is important to keep the verbal commands simple such as: sit, roll-over, stay, and come. You will find your dog learns these commands more quickly if you assign a hand signal to them as well as a verbal command. Over time, the dog will obey the command if given as a hand signal alone or a verbal command alone or both.
To teach the desired behavior either catch the animal executing it or apply appropriate but gentle pressure to get the animal into the desired position. Any abuse or perceived abuse will only initiate a power struggle between you and the dog. Once again, remembering that the learning response is the same as that of a small child.
Keeping training sessions to short time periods of twenty minutes or less and repeating them several times works better than trying to work with the animal for longer period of times.
Many trainers will tell you that the animal needs a certain amount of maturity before training can begin. Most dog obedience schools will not take an animal till it is six months old. However, it has been my experience that with gentle training the commands to sit and come can be mastered as early as 11 weeks in small dogs. Larger dogs mature more slowly and may take a little longer, but with nurturing patience you will succeed long before the dog is six months old.
If you have no experience training a dog, a dog obedience class is essential. An animal which is not controllable is a nuisance and a potential hazard. If your dog won't come when you call it, or stop, or sit, or stay, then any time it is off-leash it poses the possibility that the dog will come to harm running into a street or harm others.
Be a smart pet owner. Even if you have an old dog, today take the time to teach him a new trick. Start with "sit."