It's in the Tone...

The tone of voice that you interact with your dog in is very important.

If you heighten your tone of voice and speak quite quickly this will immediately create and excitable atomosphere for your dog, making a waggy tail and happy doggy.

I learnt this trick when working at a boarding kennels, due to noise control we had to keep the dogs barking at a low level so the owner taught me a little trick to quiten them down (granted it didn't work every time with every dog, but it made a difference), by lowering your voice as much as possible and shouting in a stern manner one word and then ignoring the situation this will eliviate the problem caused by the dog. 

When training your dog maintaining a lower voice tone and assured body language is key, especially for such simple commands as 'sit' or 'stay' as this lower tone of voice will make them feel as though they have to be motionless until you say another command.  

Also as if you act and speak excitably, your dog is going to aswell and immediately loose concentration within learning the command.

It is a very important way of communicating with your dog, as their hearing is very sensitive.

Try varying your voice tones with your dog and see the reactions,