Quote Originally Posted by polyglot View Post
Squirt a good blast of butane in the bottle to displace most of the air and keep squirting the liquid butane against the inside of the neck of the bottle for a second so that an icy crust forms, then you put the lid on. As the crust evaporates, the pressure in the bottle will go way up so you then release about half the pressure with the lid, which will get rid of most of the rest of the air. Don't let it get quite back down to atmospheric pressure - you want any osmosis/leakage to be outwards.
One further point to be added to the above.

It is best to refrain from smoking and avoid open flames while you do this.