Good article on Dental Fear Central on using 'stop signals' to communicate with your dentist, and why a 'stop signal' is not enough.
Stop Signals. A stop signal is a signal that you have agreed with your dentist which means “stop – I need a break/I’m uncomfortable/I’m feeling pain”. Some examples of stop signals are:
Read more - from http://www.dentalfearcentral.org/fears/loss-of-control/#stop