[Majost]

There's absolutely no question, zippy... you leave as much air in there as possible.

Carbon Dioxide only stays dissolved in liquid if it is pressurized. That's why new sodas release air when opening them. By keeping as much air in the bottle as possible, you allow pressure to build back up in the bottle. Although some carbon dioxide will initially be released, you'll be taking advantage of it.

If you squeeze out the extra air, you'll never pressurize the bottle to be more than the ambient pressure -- the bottle itself will just expand back out to its original shape.

Now, if it makes any noticeable difference, that's a different question. But there's no way the latter will keep it fresh longer.