I hardly ever yelled at my children (they are all grown now) but I never ever made a threat I wouldn't carry through, and they all knew that. I would inform thenm in a calm voice what the particular sanction was going to be, and then implement it if necessary.
I'm not saying I never 'lost it', we're all human, but i think if you are yelling a lot, then you need to try and stay calm and instigate a scale of sanctions, which you will then carry through. Boundaries are what instill discipline. Establish them, enforce them. Never ever give in.