Leaving the bottle out and using ice to make it cold will make the fizz last longer. Goes flat quicker in fridge. When warm, the gas inside the bottle expands more, and pushes the bottle against the cap making a better seal. Try it.
To me, the best way to drink a soda is to put it into the freezer until it gets just the tiniest bit of ice in it, then pour it into a glass glass or a stryrofoam cup. I can't stand to drink flat Coke. Not worth the calories to me.