Mix 1/2 tsp of baking soda in 4oz of water and drink*.
I'll warn you, it doesn't taste very good, but it seriously works. Both my Grandpa and my Dad have been life long devotees of this cure for an upset tum tum.
*Baking Soda is a salt, so if you suffer from hypertension or are on a salt restricted diet, give this step a miss.