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.