The usual way of storing partially-used paint tins is to turn them upside-down, so that the skin that invariably forms is at the bottom of the paint when next you use it.
Although not original, I've realised that the flexible translucent plastic milk bottles (typically 1 and 2 pint sizes) make pretty good containers for the unused paint. After decanting the paint, just carefully squeeze the bottle until the paint level reaches right up to the rim of the neck, and screw the top on (tightly). That way all air is excluded, and a skin is prevented from forming. [And don't forget to label the bottle!]
Anyone see any problems?