So you're done painting for the day and you have a ton of paint left over - mixed to exactly the correct color and shade. What to do?
At the end of a painting session, I save my oils in an old ceramic pie plate, filled with plain water.
I use a spatula to scoop the colors off the palette and store them under water in the dish.
When you're ready to paint again, scoop out the colors and put them back on your palette. Since oil and water don't mix, the water drops will evaporate and will not affect your paint or your painting.
The water seals off the paint from the air and I can keep my colors fresh for up to a week...and sometimes even longer.
I do not keep any mediums I have used.
When you "forget" and leave your paints until they harden (tisk tisk) - but you have used a ceramic or glass dish to store your paint - you can clean up hardened paint easily with a single sided razor blade.
My paint storage dish, shown above, needs a razor blade clean up as soon as that dish isn't in use - and I have nothing else to do in the studio.
* Note: You cannot store water-soluble oils under water.
If you need to keep paint even longer, you can store your piles of paint (without the water and covered of course) in your freezer.