Things that are critical:
- The area of contact between the lid and the tin must be absolutely clean on both surfaces.
- The lid itself must be completely free of dried paint.
- See Item 1.
- The lid must completely seal against the lip of the tin
Humbrol paints used to be the Volvo of hobby paints—I had tins that were thirty years old before the paint went off. Now many have reported numerous and varied problems with their quality and longevity. This last may be part of the problem.
Do not store the tins upside down. This is an old professional painters trick and it works for the short term storage of house paint. Not sure what the state of the industry is now, but back when, painters would keep leftover paint, at least of the basic colors, for the next job. But that paint wouldn't be around more than a month or two before being used. It also takes a fair amount of work to thoroughly clean the sealing surfaces of a traditional paint can. This "good enough" approach works when the paint won't be sitting around for long.