What happens when you ask your dog to sit? Does he stare at you with blank look on his mug wondering how long before you repeat it three more times and then slowly comply? Or does he immediately plop his butt down and smile up at you waiting to hear "good boy"?
Sit is one of the basic commands that all dogs should know from puppyhood. It is easy to teach, but also easy to ruin. When teaching a sit, you will lure your dog into position with a treat above his head, hold it there and step into him until he sits. After he performs the sit five or six times, name it - "sit". Immediately praise and reward him, then release him with the word "okay". The release word teaches him that he must remain in the sit until he hears the word "okay". After your dog has learned to sit, remember to only say the command once, give him three seconds to comply and without repeating the word, lure him into position. Once he understands his job, it is your job to maintain the behavior with a clear picture of what you expect each and every time you ask your dog to sit. Use a calm and reassuring voice when asking your dog to sit.
After he is 100% in compliance, start adding in "the three D's" - duration, distance and distraction. Begin with duration, start with one or two seconds and then release and reward. Build up gradually to a minute, etc. If your dog gets up, take him by the collar and place him back in the same spot he was in and resume. Set your dog up for success, if you notice that he is going to break the sit, step into him and pay him and then release him. Don't try to move too fast, build up a strong foundation, be patient, all future training depends on this beginning. Once you have a little duration, start adding in a little distance, very little to start with. And always step into to the dog to reward, never make him get up to get paid, reward in the position you are working on. Take baby steps back and go in and pay him each with each step. The final step is distraction, this takes a little longer, but your patience will be rewarded. Start with a low value toy, with your dog in a sit, set the toy down next to you, if he moves, utter a negative sound "agh" and pay him when he remains in the sit, ignoring the toy. Eventually you will increase the value of the item.
Keep your training sessions short, always end on a positive performance. If at any time you feel you are not happy with your session, stop, losing your patience and getting upset will not accomplish anything good. Train four or five times a day for no longer than five minutes, I tell my clients to train during a commercial break while watching television. Make it fun and your dog will look forward to working with you. And DO NOT repeat the command, the dog should sit when told on the first time or you will have a dog that knows he can wait until your face turns red and you are yelling "sit, sit, SIT!!!!"