I have two of my own children in Primary with me, and I've had great success with the "ice cream" technique for over a year.  (I got the idea from my mom, who had eight children - the youngest three were rowdy boys - all to herself while my dad was Bishop on the stand.)  I explained to my kids before church that I had ice cream for dessert after Sunday dinner, and they were each going to get three scoops.  But I told them if they were misbehaving at church, I would hold up three fingers to them.  That was the warning.  The next incident would be two fingers, then one finger (indicating how many scoops they had left), then a big fat zero.  They lost a lot of ice cream at first, but after just a few weeks, they caught on, and most Sundays we never get past a warning anymore!  The thing I love about this is that I can communicate silently with them - during singing time, across the pew, even from the stand.