Follow through, follow through, follow through
That is the #1 thing you HAVE to do. Now admitidly, I am not yet a mom, but I've been a nanny and a preschool teacher, so discipline is a little easier for us--we don't have to live with the kid
However, you have to be the one in control. By losing it and screaming, you are losing control and letting your kid know that they can push your buttons.
So how do you get control? Have a plan in action before anything goes wrong. Kid won't pick up the toys, you ask them in a calm collected voice, kneeling down so you're eye to eye. Explain what you want them to do, and the concequence if they don't obey. Then punishment. I like the whole "naughty corner" idea. The kid gets up, you collect them, sit them in the corner and walk away. Repeat if needed. DON'T get into a confrontation.
This is really good advise. our now DD9 is just growing out of this and it took years of sticking to our guns. And MANY time-outs for mom

When you feel yourself losing it, step out of the situation for a while and come back to it.
I give choices, but one is a consequence that I can follow through on and the other is doing what I want them to do. Stated calmly, it would be something like this: Would you rather clean your room now or miss the soccer game and do it then? I even put the "Would you rather...." on my fridge so I would remember to word it that way. Worked like a charm, because neither choice was appealing, but she knew I was willing to follow through. When she was upset later for having to stay home, I (as sincerely as possible) would sympathize "I hate it too when I have to miss something I enjoy because of a responsibility" I try not to do "If only you had...." because she is quicker to learn if she figures it out than because mom gave her a hard time.
A book if your interested that changed our discipline (I sure can't take credit) is Love and Logic (forget authors) Good luck---it will get better.