Do your kids have a "reverence reminder sign"?
After talking about what reverence is, it is good to have a sign that you can make that reminds the kids 'this is the time to be reverent'. In the Scouting program, they put two fingers in the air and then all the other Scouts, upon seeing this, quiet down and also make the sign.
I like to use music as a reminder in Primary. Maybe there could be a special song that you can sing together during prelude to remind the children to be reverent. One mentioned by Julie was Reverence is Love.
Another good one is The Chapel Doors. Or maybe you could have a 5x7 picture of Christ handy that you hold in the air and when the children see this, they are to immediately fold their arms.
After teaching the concept of reverence and using music to stress your point, have them practice the "reverence reminder sign". Tell them to talk to their neighbor, wiggle in their chairs, etc., but when you hold up the picture (or whatever sign you choose), they should immediately quiet down and fold their arms. They will enjoy practicing being quiet.
There is a great story for teaching reverence in one of the Especially For Mormon books. It is about a brother that gave his life rescuing his family in a fire. It really drives home the point of why we show reverence to the Savior.