The 'No' command has to be one of the most common commands for up and coming dog trainers. It is short easy to say and doesn't appear in any other commands so there won't be confusion. Why is this command so effective? Obviously dogs don't really know what 'No' means. Through the tone of your voice, stature and timing 'No' can be one of the most effective command in a trainers tool box.
When dogs are in their own pack they naturally don't want to fight amongst themselves, instead they growl to intimidate other dogs. When dogs growl they stand tall and rigid in order to make them selfs seem dominate especially to dogs in the crouching or lying position. When you are training saying the 'No' command is equivalent to growling at your dog. It is actually more effective if you say it in a growling tone acting like another dog in the pack. This will give you a dominate position and your dog will obey.
Along with tone, stature is another important variable to making the 'No' command work. Standing tall will help you assert your self as the dominate member of the pack. If you are lying down or sitting your dog may not obey immediately or not at all. To bypass this, sit or lay down in a common area like a couch or bed. Proceed to give your dog commands while it is also in the sit or down position. Your dog will become accustom to taking commands while you are in a less dominate position and continue to obey later on.
Finally timing is an essential aspect to the 'No' command. It must be said right as your dog is doing something wrong not a moment after. As soon as your dogs stops praise must be given, it is important to praise the moment the bad behavior stops to alleviate any confusion. Another thing to remember when using any reprimand command, never use the dogs name as a punishment. Names should only be given to gain the attention of a dog not to punish them. Always use the 'No' command when reprimanding and there shouldn't be any confusion.