Mike:
One method I have used is to premix and tube certain colors that I know I use on every painting. I have tubed a series of neutral grays from dark to light, and have tubed flesh tones which I use as a baseline and modify them to match the subject's individual skin tones. This works really well for me.
The other thing I have done in the past is to premix the entire palette for the whole painting on one of those Masterton tupperware palettes. The whole thing goes into the freezer and I take it out each day and pull colors from it onto my hand palette and then stick it right back in. I did this for my most recent painting of Anastasia which was 24x32 so I needed quite a bit of paint.
Hope that helps.
