It depends on your definition of "firm". If you have a softer dog, firm might shut them down.
I agree that all training needs to be fun. If you start out with a puppy and practice NILIF it really works and builds a great bond between you and the dog.
The first thing I really work on is focus. (For me, it's the "on me" command)
If your dog is locked in on you because they know that "all good things come" from you, you can almost whisper a command and get a response.
Clear and consistent is also important.