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?
--
Ian