In the center of our kitchen table there sits a display box full of various candles.
In the center of the box there is a tall, white candle that we use every single night at dinner. This is our manners candle. At the moment he isn't quite so tall, he is barely a little stump, but that's how the kids like him best.
You see, every night at dinner time we light the manners candle. Then, in the course of the meal the candle stays lit as long as the kids use good table manners. This includes things like keeping elbows off the table, not burping, asking politely to pass items, using forks not fingers to eat, chewing with your mouth closed (a real problem at our house...YUCK!!), keeping your napkin on your lap etc.
Since we have small people at our table, we give the kids three warnings. On the third warning we blow the candle out. No one wants to see the flame extinguished because when the candle burns completely down to where it won't light anymore the whole family gets to go out for ice cream (usually just brown toppers from our local Arctic Circle, but the kids still get pretty excited about it).
This is one of my favorite ideas ever. The only drawback is that once the kids have been awful and the candle has been blown out there is not a lot of incentive for them to start behaving because the flame is already gone. However, that VERY rarely happens and our kids have picked up some better habits at the table in the process.