How to keep drinks carbonated
Transfer the contents of the large bottle to several smaller bottles.

Fizzy drinks go flat because each time they are opened &amp; re-closed, the gas is released from the liquid until parity pressure is reached in the container, preventing any more from escaping.

Once you've reached about the halfway point in any size bottle, you're on a losing run from thereon in. Every time you release the pressure, whether you drink or not, you have a lot of gas to release from the liquid to reattain parity pressure.

If you continue to re-use the smaller bottles, the planet will send you a big green hug too ;-)