Hi Caro. Good on you for wanting to improve in this area. The problem with being a shouty parent, is that often the kids become shouty too (after all you are their role model). When everyone is shouting then no one can hear what others are really saying.
I used to get soooooooo frustrated at ds for not listening to me. I recently saw a new thing that I'd not seen before on a super nanny show, which I've started and it REALLY works (now that he listens more, I get less frustrated). Get down on your knees so that you are at eye level with your dds. Look them in the eye and calmly issue the instruction(eg, Please pick up your toys now). Make sure you make eye contact. tell them to look at you if necessary (I have to tell ds a few times before he really looks at me in the eye and listens to what I am saying) I could not believe what a difference this made to my ds. He is a bit of a dreamer and did not seem to hear much of what i said to him. Also, I had the feeling that he filtered me out when he didn't want to hear what I was saying (like background noise), as I found myself saying the same thing over and over. This technique makes sure he really does hear. Since I started using it, he has done EVERYTHING I have asked him, without any exceptions - and he has done it FIRST TIME I asked.
Another thing I find is good, is giving him a choice. I start by saying "you choose" - he then knows that what happens next is up to him. For example if he is playing up at bed time, then I calmly say "you choose, you can either go to bed, or you can go to time out" or something like "you choose - we can read book A or book B. But it is bedtime now, and you are going to sleep in the next ten minutes" or "you choose - you can have a story or you can go to bed with no story. But it is bedtime now" or "you choose - you can have teddy bear pajamas or Thomas pajamas, but we are putting on pajamas now", or (on a different topic) "you choose, you can shop with mummy like a big boy/girl and stay next to me, or if you run off then we'll have to go home (or I'll have to take you back to the car for timeout or whatever". Kids are smart. Most times they'll choose to behave because they don't want negative consequences. Giving them a choice seems to empower them. Of course they don't get to choose for example, whether or not it is bedtime, but some other choice, eg of pajamas or story, makes them feel in control and less likely to act up.
The choice thing is great, because it takes the pressure off. If they choose the behaviour that you don't want, then they choose the consequence they don't want. But there is no need to get worked up about it because you and they both know what the consequence is going to be in advance, because you have told them. I find this helps me stay calm.
Sounds like you have great kids by the way