Have you ever watched the tv shows "Nanny 911" or "Super Nanny"? They are absolutely fantastic shows about real life families in crisis like this where the children are out of control and the parents don't know what to do. They send in professional counselors to help them get things sorted out. I would suggest trying to watch some of those shows. You will learn a lot from them.
The answer to your question is nothing short and sweet. It really depends on A LOT of things. I think that the most important factor in regaining control of your children is learning proper communication. I do not know how things go in your household, but here are some tips...
Screaming and yelling is a terrible way to communicate. You must try to maintain composure when dealing with an unruly child because children learn by example. Try to get on their level and talk calmly and clearly, but in an ascertive tone.
Also, there must be consequences for bad behavior and you have to stick with them. Do not allow yourself to be sucked into negotiations either. Children are really good at that. Children need to have discipline. Make them sit in a time out area for a designated time and tell them that they cannot leave that area until they give you a heartfelt apology. Make sure they know why they are there too.
You can also take things away as a punishment. Such as games, tv, play time, etc.
It is also a known fact that children who are actively participating in mentally stimulating projects are much calmer and content. Children need your love and affection and some times the best way to do that is to do special things with them. Some ideas are to take them to the park and play with them, take them to the zoo, do art projects together, show interest in some things they are into, etc. A lot of parents have a tendency to leave their children to their own devices (creativity) and expect them to be self-sustaining. Make your child a 24/7 part of your life by having him help you cook meals, help you do laundry, help with yard work, etc. The key is that you do it together rather then make him do it alone.
One thing that I saw on those shows that seemed to help a lot, was that they had the child start either a diary or a little suggestion box. It gave the child an opportunity to express his feelings towards the parents in a non-confrontational manner. I was suprised that the children seemed to really like it. The important thing though, is that you do not have any harsh or angry responses to what the child is saying or they won't use it anymore. This will open a line of communication which is really what you need. It's also important that you read them on a regular basis so the child takes it seriously and knows that you are really interested in their feelings.
Just remember, anger begets anger! Winning back your childs respect and communication is no easy or fast process and takes a lot of hard work and tears. You must be strong, enforce your rules with no hesitation or variations, and take an active role in your child's activities.
There is a lot more to this situation, but I would be writing a novel. I hope this helps!