I practiced a good parenting strategy today and thought I'd share. (You don't read much about my bad moments--usually all there is to learn from them is that Mom needs more sleep and/or self-control!)
The four-year old was weeping loudly at the table because she wasn't sitting next to her chosen cousin. While I was still calm, I crouched down to Becca's level and told her, "It's okay to be sad and it's okay to cry, but you may not cry like this at the table. If you need to finish crying, you can go up to your room or to the family room. Do you need to cry some more?" Either yes or no would have been fine. She decided to stay with us.
I've used this technique with all my children, mostly when they were young. I think it's important to validate the emotion (sad, angry, disappointed, etc.) but not allow disruptive behavior (crying, screaming) in a family group setting (mealtime, prayer time). I will repeat that it's okay to feel [whatever], because I truly believe it. Emotions are what they are, and being little can be tough.
I try to deal with the acting out right away while I can respond not just calmly, but kindly. It's no fun to feel bad.
If she can't regain control, I'll send or lead (depending on age) the child to her chosen "other spot", and maybe even stay for a few minutes. She comes back after calming down. Sometimes he'll try to come back before he's really ready, and we'll need to repeat the process, but this strategy has worked well for our family.