It is so worth it though as there is nothing better than having all of your paint premixed in clean little tubes so that you can squish out fresh paint every time you paint. I have thought of mixing while sipping a glass wine or martini but I am sure I would end up with very sub par mixes ;)
I don't premix all my colors but I find it so helpful to premix a flesh palette as I may not get the chance to paint every day, especially in the summer, and there is nothing worse than mixing up your paint and then having to through it all out because it did not keep between sessions.
Also, as some days I only have an hour to two to paint and if I have all of my colors premixed, I get to spend almost all of the time painting rather than getting ready to paint. Of course, even when premixing the mixes are merely starting points but they create a sense of harmony and consistency.
Tubing is not for everyone though so I listed a few other ways to keep your paint usable between sessions. Oil paint does not actually dry out, it oxides due to contact with the air. So the key is to slow down the oxidization by limiting the amount of air the oil is exposed to. I have tried all the methods below and they all work, depending on the size of paint nugget left and the amount of time the paint is left out. I listed them in order of my preference after tubing as nothing compares to fresh paint ;)
I use this method for plein air painting and the first time I tried it I used too much clove oil and my paint was a little runny so I just use a little oil now. I also use it in the studio now (duh, I don't know why I didn't sooner) as I like this method because I paint mostly using the indirect method which uses smaller piles of paint and I find that it prevents a skin from forming more so than freezing.
I like the smell of clove oil but it can get pretty strong so when I am painting, I remove the q-tip and put it in a zip lock bag while I am painting and then just put it back on the palette after my session. You can also add the oil directly to the paint (1 drop per 1 inch of paint squeezed from the tube) but I don't like to do that as I like my paint to dry once rather quickly during painting so I can paint the next day if I want to...and I always want to ;)
The only issue with this method is that if you forget to refill the water and the paint dries out...yeah, so my paint is still stuck to my palette...I will have to scrape that off at some point ;)