Sometimes it's difficult to finish drinking a large bottle of soda before it goes flat. To keep soda carbonated, prevent as much of the carbon dioxide from escaping as possible. The more it's exposed to air, and the higher the temperature at which the soda is stored, the more carbonation is lost. You can also keep sodas from going flat by using special products to serve or store soda to minimize carbon dioxide loss.
Air, Temperature and Freshness
To minimize the loss of carbon dioxide, the gas that gives soda its fizz, cap your soda bottle and tighten the lid firmly as soon as you open it. Store the soda in the refrigerator or on ice to keep it constantly cold. The colder the temperature, the easier it is to keep gas dissolved in a solution. The fresher your soda, the better it will taste, so check the "use-by" date on the side of the bottle.
Special dispensers are designed to maintain the fizziness of soda. Dispensers that turn the bottle upside down keep soda fresh by preventing the carbon dioxide from escaping. Carbon dioxide escapes upwards, and the upside-down orientation prevents it from doing so. Some specialized bottle stoppers are designed to re-pressurize opened bottles, helping preserve carbonation.