Toothpastes formulated for sensitive teeth usually contain either potassium nitrate or strontium chloride. These chemicals plug up the tubules in dentin [source: Zamosky]. Some travel all the way down to your teeth nerves, where they build a shield. Strontium chloride in particular triggers minerals in your saliva to harden over the tubules [source: Save Your Smile].
Brush these pastes on with a soft-bristled toothbrush (avoid hard bristles because they scratch enamel over time) or even a finger if your tooth is super-sensitive. Brush or rub gently. Give the chemicals time to interact before spitting, and wait a few minutes before rinsing lightly. Sensitivity should disappear in two or three weeks.
If new toothpaste isn't enough, add fluoride to your routine.