I've tried to explain this with a set of Back of the Napkin style pictures as you will see below.
Now, the above picture is quite self explanatory. There's a good reason why you need to use a firm tone of voice -- it establishes your role as the Alpha and illustrates your confidence. Its also a way to avoid the dog from interpreting your command otherwise. Remember the dog doesnt understand your words; it merely recognizes the tone.
Step 1 (Situation dependent):
Some dogs dont react to the treat being held over their head. In such cases the above step is applicable. Most dogs would react to the situation described in the previous picture and automatically sit in anticipation of the treat.
I'd like to talk about the concept of markers here for a while. A marker, such as the sound of a clicker or you saying "Yes!" each time your dog does "the right thing" is used to acknowledge appropriate behaviour. Remember to be consistent with your markers! i.e. Dont say "Yes!" once and "Hurray" the next time. Markers also help you build the bond with your dog as it starts to identify you as a fair person (who rewards when the behaviour is appropriate). The key to setting markers is to use them as soon as the right behaviour occurs. In our case, its when the dog's butt touches the ground.
The final step is one that should never be neglected. This is where you reward the dog by giving it one or more of the following:
- toy (for dog's with a strong play drive);
- food (for dog's with a strong food drive);
- praise (give loads of it - dog's can sense the encouragement in your voice);
Remember that when petting - pet gently! Its very easy for a dog to get over excited and lose focus if you subject it to heavy petting.
Repeat this over and over again and you'll have an expert sitter. Remember to do this under distracting conditions as well and once you're sure the dog knows what to do, give it a correction if it errs. Lastly, do keep in mind the Leerburg rule of 30 - a dog needs to perform an action 30 to 40 times to internalize and learn it. So repeat, reinforce and reward.