Drinking cold peppermint water works instantly for me whenever I get motion sickness or nausea from eating bad food. Also, whenever I'm chained to the bathroom because of the flu, peppermint water helps to keep me from vomiting.
One or two drops of Peppermint extract in water. The ratio of peppermint to water really doesn't matter- I generally like my peppermint water strong (1 drop of peppermint in 3 table spoons of water), but everyone else in my family prefers diluted stuff (2 drops in 6 ounces). Nevertheless, it will still work.
Peppermint extract is available in the spice section of most of the grocery stores I go to.