you exhale into the bottle just before you close it,,,,,,or give a squirt of Nitrogen from a can
human exhale is a large percentage of Nitrogen with Carbon Dioxide as the second gas,,,,,Air is the same percentage of Nitrogen with "bondable" Oxygen as the second gas (second by volume in both cases)
If you haven't been exhaling/squirting,,,then you have had free air entering and sealed into your paint bottles since you opened them the first time,,,,,and air is what causes the paint to dry inside the bottles
when I switched to Acrylics three years ago,,,,,,I gave away over a hundred Model Master Enamels (all still good at inspection), some of which I bought in a Wisconsin hobby shop before it shut down when I lived up there,,,,,,,I moved to Tenn from Wis in 1992
as with cleaning brushes and tools,,,,,,,I have found the "my stuff doesn't last as long as his" posts to match up with "I don't do any of that fancy stuff" posts,,,,most of the time caused simply by not knowing that there are things that you can do,,,,,,and believing that they can actually work if tried
another is the "shaking kills paint" posts,,,,,,yup, it will, if you open the jar, leave it open as you paint, close the lid nice and tight, and then shake it just before you open it up for the next use,,,,,,you will have perfectly aerated paint that you see has dried out after you pry the cap off with Channel Locks
but, if you get a proper cap, clean and wax the threads on the jar, exhale into the jar before putting the lid on, and use a non-reactive and non breakable bead,,,,,,you can shake that bottle every time just before you open it,,,,,,,you shake today's bottles, let them settle out the bubbles (which aren't air anymore), and then stir each slowly with a stir stick before decanting into a palette or airbrush cup, exhaling and covering the bottle immediately (proper caps seal along the *inside* of the jar edge, not the outside as they are sold to us)