Leave it as it is or, if possible, increase the pressure. When the bottle is squashed, the plastic starts to bend back ino shape once let go of, increasing the volume while the mass of air stays constant because the lid is on. When there is less pressure pushing the bubbles down the drink becomes flat faster.

The previous answer is correct, but to elaborate a bit:

The fizziness is carbonic acid. This is formed by a reversible reaction between carbon dioxide and water:

carbon dioxide + water <---> carbonic acid

All reversible reactions have an equilibrium position, when the reaction happens at the same speed in both directions and therefore the amount of reactant in both phases remains the same. Equilibria are stable until conditions change. In the reaction above, the equilibrium is between carbon dioxide pressure and carbonic acid concentration.

When you take the top off, you release carbon dioxide and reduce its pressure. The chemical reaction proceeds in the left hand direction to try to reach a new equilibrium position. The reaction is not instantaneous, so it takes time 

As long as the top is off the carbon dioxide pressure cannot increase, so the reaction continues going to the left until no carbonic acid is left. This is what makes fizzy drink go flat.

With the top back on, the carbon dioxide pressure starts to increase. Pressurising the bottle helps, although not as much a you might think because it is the 'partial pressure' of carbon dioxide that affects the equilibrium position, not the total pressure, so the amount of carbon dioxide that has to come out of solution to produce this is not as different as you might think. However, at a given temperature the pressure x the volume is a constant. This means that if you squash the bottle flat, the bottle may expand without changing pressure until it reaches its maximum volume, at which point the pressure starts to increase. However, because the gas volume in this case will be 100% carbon dioxide, the cases are not as different as you might think - if there is an initial air volume, carbon dioxide gas still has to evolve until the partial pressure is sufficient to reach the equilibrium position.

I guess this counts as a fairly pedantic answer, although i have left out the maths (it's a great problem for chemistry students, though)

I noticed some unscientific holes in my answer. First I don't know if the manufacturers add a charge to pop bottles to clamp down on the CO2 in solution or what that psi value is. I also don't know if that initial charge does force the CO2 back into solution. When you shake an unopen bottle, bubbles form but then seem to get reabsorbed. But is this really true or does the pressure within the bottle increase indicating CO2 has left the pop never to return. I'm also wondering if there really is any such thing as an equilibrium pressure. If there were, it should be a constant whether the bottle is 3/4 full, half full or 1/4 full. It should be constant because the surface area is constant. But this is not the case because you can hear the pressure difference at different levels of liquid. It seems the more solution containing CO2, the more CO2 can escape regardless of an equilibrium pressure. Just shake a bottle at equilibrium and more gas will escape. Maybe the equilibrium is a function of time and not of pressure at all. Yet my external bottle clamp really does work which means pressure does play a role. Ok, now I don't know the answer at all.

Yes I am now convinced, without any scientific data to back it up, that the only effect pressure has is dampening the movement within the liquid that causes the gas to escape. There is probably a high pressure value that does prevent any gas from escaping but that value is irrelevant once the bottle is opened because it can not be reached by the gas in solution. Hence, low pressure allows more time for gas to escape because the internal movement within the liquid goes on for a longer time. The higher the internal pressure, the less the movement and the less gas escapes. However, by shaking the bottle, more gas escapes which increases the dampening pressure which is much higher than the "equilibrium" reached before the bottle was shaken. Also, by crushing the bottle you are effectively reducing the area of the liquid and hence less pressure is needed to maintain the same dampening psi. So I'm proposing there is no such thing as a fixed equilibrium pressure (partial or not) once the bottle is opened that can prevent gas from escaping. The amount of gas that escapes is a function of how quickly the pressure dampens movement within the liquid. So the answer is yes, crushing the bottle slowly (to reduce gas escaping the liquid and pushing against the plastic) will keep the pop better carbonated by reducing the area of the liquid and hence requiring less gas to create an adequate dampeing pressure. Unfortunately my brain was beer carbonated at the time of writing so I don't know if I made sense yet.