I place my paints in very small (lunch box to slightly larger) air-tight plastic clear containers. I've heard of the "up-side-down" method (but I think this is silly). Other ideas i've contemplated of doing (but never done it) is place a strip of sticky tape around the edge of wear the jar & lid meets (but it would have to be wide & sitcky enough but this doesnt work for little tinlets) or is placing a glad wrap or plastic sheet over the top of an open tin & push it inside so the it pushes the extra air out & close the lid with the plastic flapping out the top with the lid on.
Other than looking after your paints (ie keepin in recommended paint temperature ranges) thats all i can offer!
Remember - paint has a shelf life of about 10-15yrs or even 20 - but that depends on the type of paint, the manufacture & that specific "batch" of paint. By time you buy a paint jar, it may be already old by a couple of years!!!! Nothing lasts forever!