I try to determine from the ruff the elements that form the foundation of the mix. Alot of times this can be the drums, it can be the guitars, or any number of things. Let's say it is a rock song, i would probably start with the guitars, then leads and backs, then drums, then go back and work on vocals some more, then put in all the little candy parts. On a pop or hip hop record I would start on drums and bass, then put in pads, and foundational keys and guitars, the vocals, then finish up. If I only have two or three hours for the mix, I always start with the vocals. I always study the ruff mix. Nowadays people spend a lot of time on them and they can tell you a lot of information. If you are the guy doing the ruff, then follow the music in your head. It is kinda like building a house. you don't start with the roof, you start with the foundation!