If you're in a situation where something is setting off your gag reflex (dentist's offices are one of the most common places), just form a fist with your left hand, squeezing your thumb. The folks over at Reddit's Ask Science forum deliberate as to why this is, and they point to a 2008 study that shows it to be effective—not to mention lots of anecdotal evidence on the internet—but whether it's an effect of the nerves in your hand or just a distraction, it looks like it's a pretty useful trick.
Lifehacker reports: 
