I too have become a yeller and hate it. I've started taking a parenting class called "Redirecting Children's Behavior" and already within a week I've seen remarkable changes with my son and amazingly enough, I'm not yelling. The first thing I had to accept was that - Misbehavior does not equal a bad child or bad parenting. Misbehavior is a child's way of communicating his/her needs and it is my job to help them figure out what those needs are. Second, we are constantly telling our kids what to do. Studies show that in one day a child can receive upwards of 1,000 commands. Now if someone was telling me what to do, how to do it, etc. that much at work, I know I would go crazy too. Kids start to tune you out just as you would likely do to your boss.
This is where you have to involve your kids by asking them how to solve the problem at hand and you have to allow for natural consequences and let them make mistakes. We need to raise kids who can think for themselves and understand the consequences of their actions. For example, it's time to go to school and they don't want to wear a jacket. Fine, let them go without and if they get cold, trust me, the next day they'll take a jacket. They don't want to eat, fine, they'll get hungry and eat the next time. You get the idea. The important thing here is not to say "I told you so." If they complain about the consequence they brought on themselves, just say "That must have been tought to experience "xyz," I'm sure you'll think about that when you make your decision the next time.
If they don't want to turn off the TV, go to bed, etc. say - we have a problem here...I need you to turn off the TV. How are we going to accomplish that and what do you think the consequence should be if you don't - you may have to wait a few minutes for the response but let them think it out. This is where I'm amazed that my son decides he wants to what I've requested as opposed to figuring out what should happen to him. Now this approach takes more time, but I'm yelling less and less and my son is a lot happier.
Also, a lot of misbehavior comes from your kids wanting your attention. So strive for "genuine encounter moments" - where you focus on them. Put down the phone, computer, etc. and look them in the eyes and give them your attention. If you can't do it right that moment, let them know when you can and then follow through. I've also found that just rubbing my son's back while I'm dealing with his sister is enough attention to ward off the misbehavior as well.
One final trick is the "pause" button technique. If you feel yourself getting overwhelmed, put you hand on your forehead, breath deeply and "pause." The more time you can put between the "stimulus" that is triggering you and your "reaction", this can help diffuse the situation and your impulse to yell.
I hope this helps you - it's certainly helping me!