7-up is lemon/lime flavored, easy to digest, and colorless so it doesn't stain when you spill it or throw it up again. I don't know whether its acidity or carbonation help, though lemons and limes are also anti-nausea remedies (limes are also delicious and prevent scurvy!)

However, in my experience Coke/Pepsi (phosphoric acid) and ginger beer/ale (ginger extract) may work better as anti-nausea medicines.

Canada Dry was sued for their Ginger Ale not containing "real ginger" (as per the label), but I think it still contains something like ginger extract. The anti-nausea effects may be one reason airlines serve ginger ale as an in-flight beverage.