There are a number of strategies for keeping oil paint from drying out on the palette. One is to put it into the freezer. Another is to submerge the paint under water (before using it again, you drain the water away and let any remaining drops evaporate). You can also cover your palette in plastic wrap to seal out oxygen, but that gets kind of messy. Some artists squeeze out a large blob of paint, then let it skin over. When they want to paint again, they cut the skin away with a knife and use the fresher paint inside.
The best paint is fresh paint. In a perfect world, you would have paint made fresh every morning, but that isn’t practical unless you have an indentured servant to wake up at 5:00 AM to mull the day’s paint. The next best paint is fresh from the tube. Fresh paint has the most binding power and the best handling. Old paint is sticky. Paint that has been frozen and re-thawed has undergone chemical changes—even if it seems OK, it isn’t quite the same stuff. Paint that’s been put under water may absorb some of the water and that also has the potential to cause problems.
Most of the time, I squeeze out only the paint I think I’ll use that day and discard any unused paint rather than trying to save it. I’ll often save it from one day to the next, but no more than that. I don’t waste a lot of paint, because I don’t put really large blobs of it on my palette anyway. When I need more, I squeeze out more. I’ve seen advice that says you should always have lots and lots of each color of paint ready on your palette, because that somehow makes you freer and more creative. I don’t do that, and I think I’m plenty creative. The worry seems to be that unless you’ve got big honking wads of paint right in front of you’ll be too restrained. Worse yet, you might use the wrong color rather than get up and get more paint. I’ve never done that, because squeezing out more paint isn’t really any effort.
So I am a little stingy with how much paint I put on the palette and always willing to squeeze out more on when I need it. Works for me.
Every once in a while, there is a reason to save paint for a few extra days. That happens sometimes when I’ve made some paint up fresh or when I’ve put a lot of work into mixing just the right color. I think the best way to save oil paint is by retarding its drying rate with clove oil. Clove oil slows the drying of oil paint without, so far as I can tell, causing significant chemical changes or causing any stickiness. I don’t like to mix clove oil with my paint, because that will retard its drying after its been applied to the painting. I have heard of one artist who has lots of small glass jars. She saves paint by smearing some clove oil inside a each jar and then putting it upside down over a paint blob on a glass palette. What I do is similar: I transfer my paint to a ceramic butcher’s tray, then smear some clove oil around the sides of the tray and cover it with plastic wrap. Most paint will last an extra few days this way. If you like to paint in one layer and you like to play around with wet paint on the surface for days at a time, mix a drop of clove oil into each nut of paint on the palette. It will stay workable for a long time.