The Artist's Handbook, by Ralph Mayer, is considered by many to be the bible of how to work with many types of art supplies, including oil paint.
Here's a caution for anyone who saves paint (using any method) and then finds that it has turned even slightly sticky. Mayer makes it clear that once paint has begun to turn tacky or viscous, no amount of thinner, oil or medium used to "reconstitute" it will reverse the chemical reaction of oxidation that has begun.
Oil paintings created with paint that has been allowed to dry, even a little bit, and then thinned out to match its original consistency are much more likely to flake or blister off the canvas. So, regardless of what method you use to store paint, be sure it's still pretty much the same consistency as when you squeezed it out of the tube and don't try to thin it out again.
By the way, here's my method for saving paint: I can keep it fresh for a few days in little "snack size" ziplock bags, making sure to squeeze all the air out. When I need the paint again I just snip off the corner of the bag and squeeze the paint back out on to my palette.
If I don't have large enough piles of separate colors that I want to keep, but have a pretty good mess of different colors, I sometimes mix them all together and save the result as a fairly useful neutral. It harmonizes with my current painting since it contains all the colors of the painting I'm working on.
I mix this neutral with colors straight from the tube to de-saturate them, or to cool down a warm color, for example. (I work with a very limited palette, so the mixed-up neutral doesn't contain impurities of too many different pigments and isn't too muddy.)