Separate sessions for each song for tracking.
I mix one or two songs, and base the rest of the record off of those, but...
The more projects I do, the less I'm worried about consistency in the form of "every song sounding like the same mix," and worry more about each song sounding as great as it can, and if the album works together as a whole.
Some songs just call for different tracking techniques, tones, etc. At that point, they'll never sound exactly the same as the rest of the record...and I don't think they should. I think, if the material on the record doesn't all sound like the same song (and hopefully it doesn't), then each song will need to be mixed on its own, focusing on making that specific song sound the best it can.
Just my $0.02