I have used thousands of cans to paint architectural models. Often many cans sitting around for years. The longer the can has been sitting the longer it needs shaking, but shake it at least a full 2 minutes minimum. Some pigments are harder to mix back in than others.

If the can has lost a bit of pressure, heat it up in a container of hot water.

Also painting on too hot or cold days will change texture a bit. Often I was after a textured finish for brick & roads, so would use 3 or 4 colours, spray from a distance to get a grainy finish.

I often stored them on their side like wine, but that was simply to see the label on top of the can as they were not coloured.

Touch up paint will get increasing harder over days so if you need to do anything between or after coats let the paint harden accordingly.