Set everything up ahead of time

When I worked with Barrie Maguire, I totally embraced the idea of setting up everything ahead of time. Go in a day early. Pay for the time yourself if you have to, but get everything set up and checked before anyone else shows up. Label everything, set up a guest station, get your DAW template squared away so you can start a new track in 45 seconds or less. Everything needs to be routed and bussed in a convenient way so that anything that comes up during the session can be handled quickly and easily without disturbing anything else. Make absolutely sure you can monitor the headphone mixes you're sending out without having to go out to the tracking room. Don't share mics between setups if you can avoid it - If there's only one 67 and you want it on both voice and guitar, just pick one and choose something else for the other. It matters less what mic it is compared to the inconvenience of having to move mics, cables, settings, routing, etc. when someone is trying to be creative. It may appear to be no big deal - just move the mic - but when you change songs, fix parts, do overdubs, it gets hairy very quickly and you'll spend too much time getting the sound back.