Over at Terese Nielsen's blog, about a year ago (which tells you when the bulk of this post was written, which has been waiting for a busy week like this one to bring out), she started talking about the other end of the oil painter's complaint-spectrum: how to stop your oils from drying, specifically, drying on your palette. Usually, artists are concerned with getting their painted oils to dry as fast as possible, and I've addressed how I go about doing that in other posts.
But, while you want your oil paint to dry as soon as possible once you're ready for it to dry, you typically want the paint on your palette to stay wet basically forever. In fact, small oil mixtures on your palette can start to become unusable within hours, depending on how little you mixed and what pigments you used. If you're like me, not mixing large batches of each color, but mixing or altering many things on the fly just using your brush to rub colors together on your palette, then you'll find that these little bits of mixed colors dry very quickly. A day after you squeeze out your colors, the fastest-drying ones develop a skin you have to peel off to get to the wet stuff beneath, which is also already less wet.
Currently I just don't squeeze out a ton of paint at a time, and mix Walnut Oil into the fast-drying colors. Walnut Oil is slow-drying, but not crazy-slow. It stops skinning on slow-drying colors for about 2 days. When you use this paint it does dry a little slower on the board too, but if you're using a drying medium while you paint, the Walnut Oil doesn't get in its way very much and the paint still dries pretty fast. Plus, there's my whole oven-thing, and its addendum.
Terese mentions storing your palette in the freezer when not in use. I'll have to try that, although having painted out-of-doors in freezing temperatures, it seems that paint gets a little gummy when it starts to freeze--I suppose she means that it reverts to proper viscosity after thawing. There's also submerging your palette in water, which a painting instructor swore by, but I use disposable paper palette, so that's out of the question.
I've found that paint's visceral properties change as they start to dry on the palette--a slow drying color might take 4-5 days to form a skin, and the wet stuff will still be beneath that. But it's been slowly drying the whole time, from the moment it's squeezed out. Oil dries by the molecules slowly binding after the release of oxygen. Even a day later, oil paint is already stringier, thicker, more rubbery. It doesn't pull as well or blend as well as stuff that's just a few hours or even a workday old. Even paint with Walnut Oil added in is still drying 2 days later, and when I finally finish a blob of paint after 3 days and squeeze out some more, that new batch feels so much better.
So, in the end, I'd recommend just squeezing out less and adding more as you go--or just using up whatever you put out quickly. Some Walnut Oil can preserve the wet feel of paint another day or two, but use it up ASAP. Just know that you'll fall under Rockwell's curse. He once visited another illustrator and described his experience doing a little demo-painting for him. He noticed that that painter had only squeezed out small amounts of paint, and this annoyed Rockwell, who would squeeze out very generous amounts and probably dumped a lot of it. He was annoyed at this apparent frugality and stated that he firmly believed this was a key insight as to why the illustrator was not very successful. Yow!