This article is from the Miniatures Painting FAQ, by Brenda Klein email@example.com with numerous contributions by others.
Shake or stir them often, put plastic wrap between the cap and bottle
on paints that come in glass jars. Acrylics reconstitute fairly well
with the addition of water and a good stirring. Oil-based do same
with thinner. Try and keep your paints in a place where temperature
remains fairly stable.
Users of both Polly S and Humbrol have had good results from storing
their paint upside-down. The paint itself augments the seal and
keeps all air out.