I try several things, beginning with what I have already.
Like alphagirl below, I think ginger is a wonderful remedy for an upset tummy. Either use her recipe or get it in another form. I try to keep pickled or crystalized ginger on hand just in case. These can be bought in an Asian market.
I also use ginger beer. By this, I'm not referring to ginger ale. Get the good Jamaican ginger beer that has a bite to it. Again, you will probably only find this at a Whole Foods type place. This remedy does double duty for me. The carbonation helps me burp, which also seems to relieve me when my stomach is upset.
With that said, I also try Coke or Sprite with soda crackers. This was my grandma's favorite way to cure an upset stomach. Another thing she also tried was baking soda mixed in with water. Use about half a teaspoon in 8 oz. lukewarm water and drink.