Submission should be like sitting down.

When I say submission should be like sitting down, I am referencing to your dog.  How you practice making your dog sit down is the same way you should practice teaching your dog to be submissive.  This rule applies even more when you deal with aggressive breeds like Pits, Cane Corso's and other dominant breeds.  The best way is to teach your dog to lay on there side and practice making them lay on there sides in any environment.  This will make them feel vulnerable and they will have to trust you for protection. Good luck.