Gag Reflex

A month or two ago M starting gagging whenever she overstuffed her mouth. Today she did it while we were having lunch at this amazing Sushi/Dim Sum Buffet. Only problem was that this time she went too far and actually emptied her stomach all over. Thankfully I had a cloth diaper in the diaper bag and I was able to catch a substantial amount. However, the stuff still reeks! Once we could get her to calm down we got some rice and soup into her so she wasn't running on empty. It's so sad to watch her lose it all. She doesn't understand what her poor body is doing or why it won't stop. Silly thing about the situation though is that it's completely mental. How do you teach a toddler to turn off the gag reflex?