Ok, ive not finished reading all the replies to my boyfriends first post, but i realise theres some confusion over what I do…
I drink a glass of coke, and then with the top off the bottle I squeeze the liquid as far up the bottle as possible and replace the lid. Therefore leaving a very squashed bottle of coke, with only as small amount of air (as is possible) at the top.
Ok convince me this doesnt work!! (ill read all the replies now)
Reading the earlier replies should clarify why your carbonation-preservation method won’t work. But if you were drinking wine rather than Coke, your method would help prevent oxidation of the remaining amount due to reduced volume of air in the bottle (and surface area of liquid in contact with the air). [snob]Of course, if you were drinking wine from plastic bottles, you’ve got bigger problems than a little oxidation.[/snob]
A Goldbergian alternative which doesn’t seem to have been suggested so far is to squeeze the bottle, then put a big ol’ clamp on it so it can’t return to its original shape. Of course, then you’d end up with a big ol’ clamp taking up space in your fridge.
Personally, I’d suggest just drinking all the Coke before it goes flat, and using the extra ingested carbonation to conduct belching contests with your boyfriend. It’ll be a bonding experience.
Actually, since it may be the diffusion co-efficient that is important, my experiment might have been invalid. While the CO content of creamy soda is larger than that of coke, its diffusion rate is noticablely lower. Creamy soda stays far more carbonated under normal use. I guess someone else is going to have to take up the mantle as I don’t go out of my way to drink coke. Seeing as mattfjohnsonuk and co drink coke on a regular basis, I think they are prefectly suited to take up this next phase of experimentation 
No, think of it this way, vigourously shake a bottle of coke and then open it and allow to come into equilibrium, pour a drink and see how flat it is. FAR more CO2 escapes due to shaking than normal.
A quick and dirty way of doing it would be to attach a fairly thick balloon to the end of the coke bottle and measure the size when at equilibrium. Then, you can hook up a pressure gauge to the balloon at the same size and get a rough idea as to the pressure. You don’t need to get it exactly right as long as you can find the pressure co-efficient of the balloon.
Right now, about $1.20 USD on special.
sorry, did not read all the posts
I have poured about 1/3 of a 2L bottle, squeezed the bottle until the soda was less than 1/2" from the top, capped the bottle, and then put the bottle under lots of pressure form two clamps.
ALSO,
just tried the clamps on an undeformed bottle about 2/3 full.
It made no damn difference as far as drinking quality is concerned - just try to finish it, but since a 2L is about half the price of the 20 ounce bottle, WTH?
I suppose I’m the only one who thought it said “jizz” instead.
Just thought I’d throw another variable in for y’all. If you do buy 2l. sodas and aren’t going to finish them right away. Be sure to get them cold in the frig. before you open it. Then after you pour what you want, cap it and put it back in the box. A hot soda opened first and then put in the box’ll go flat quick.