L.B.
As much as we would like to sit our kids down and have an adult-like conversation with them, that is just unrealistic. I would highly suggest looking into the 1-2-3 Magic parenting program. Most libraries have the books & video tapes and you can call your local Parents as Teachers Coordinator to see if any classes are being offered locally.
The program is quick and effective and works almost immediately. I've taken 1-2-3 Magic, Love & Logic, and Common Sense Parenting...eventually I ended up using a little of everything but to just get started and stop the atrocious behavior I would recommend the 1-2-3 Magic. The principle is that when the bad behavior starts you begin counting, "That's One" if it continues for another 10 seconds you count "That's Two," and if it again continues, "That's Three, take xxx (minutes time out - 1 min per year old)" Now, the big key here is that a) the time out doesn't start until AFTER the tantrum or whatever has ended and b) there is no discussion about the behavior before or after. Black and white - this behavior is not tolerated, if you do this then time outs will occur. Time outs will continue to occur until the behavior has been modified. I have to say I saw a near immediate change in my kids...Within a week they were downright angelic! LOL... be patient and firm and make sure everyone is on the same sheet of music!!!