Because I tend to work on my paintings a little at a time most days, rather than in big blocks of time, I need a method to keep the oils on the palette from hardening. I like to squeeze out a decent amount and really do not want to throw any of it away after just an hour or so of painting.
Here is my solution to the problem. I have a Daler Rowney Stay Wet palette intended for acrylics, which really doesn't see any acrylic action these days. The well is 14 x 10", so I cut a board to fit inside the well and cover the board with freezer paper.
Because the paper is bright white I wipe some burnt umber alkyd paint all over it to reduce the white and to help make colour mixing more accurate. It dries in a trice and in the meantime, you can draw out the composition.
When I finish the day's painting I pop the lid back on the palette and put it in the refridgerator and the paints don't dry up as quick as they would if they were just left out in my work area. I have read that some people put a drop or two of clove oil on a cotton wool bud and pop that inside as well, but I don't find the need to do that.
The freezer paper can be cleaned off a few times, I tend to wipe up the centre mixing area, leaving the tube paint in piles, but eventually the palette needs a clean up. Instead of scrubbing away at a traditional wooden palette like I used to, I now just throw away the old used freezer paper and put on a fresh one. This works perfectly for my busy life.