I've already done a post about mixing on the solo bus. That's worth digging in to but it was more about the gear than a mixing philosophy. Today I want to go over ways that you can use the solo bus to improve your mix and speed up your time to finish. This should apply whether you're mixing live or in the studio.
When mixing live you sort of have to use this technique already. Going through a sound check a channel at a time you have to think about how it's all going to fit together in the mix when you're done and pretty much nail the gain structure and EQ the first time around. Even if you already know the act this can still be a challenge and it's what separates the good live engineers from the run-of-the-mill.
As you're going along hopefully you're EQing the kick and bass so they can coexist in the mix. You might hear a chunky guitar and take some bottom off so it has its own place to live but remember that it's there in case things sound hollow. You might carve out an acoustic guitar so it sits well against electric guitars and vocals. Backing vocals might get a less clear EQ so they don't overpower the lead. All these things become second nature after a while.
Once everything is up and running and the band is playing it's a good idea to do a sweep of each channel. Slap the cans on and listen back and forth between what you hear in the room and what you hear on each soloed channel. There might be something missing or you might find that there's something there that you can take out without harming the mix. On the bass channel for example, if it's nice and fat and the material doesn't warrant any popping and slapping, you could easily roll down the highs without hurting the bass and free up a little headroom and clarity for things like guitars and vocals. If you hear a big clappy mid on the kick that isn't contributing to the sound, take it out and make even more room. Just make sure you go back and forth so you don't wind up hurting the mix by getting too surgical.
In the studio it's tempting as well to go channel by channel and tweak each one to the nines before you ever even start to mix. Some of the best cats in the industry don't do a thing until they've listened through a time or two and just mixed what was there. It's a shame to EQ and process the life blood out of something just because that's what you always do. You might be working on something that's unconventional or maybe just has one or two unconventional inputs. That's where the genius of the players can shine through, but only if the mixer has his thinking cap on and isn't afraid to stray from the norm.
So let's say you've put up a mix on your DAW and have levels set. Then start soloing channels and see what you think ought to change. You might find that a mid you would normally cut might leave a hole in the mix and is better left in or treated in some other way. Dynamic processing and effects should be handled this way too. Hear the song as a whole, then hear each instrument group or "stem" as they like to call it and make a few changes, then go in to the individual channels. You'll be surprised at how differently you wind up treating things when you keep comparing your changes to the whole mix.
I did a post pretty early on about an alternate method for getting a mix together in a hurry. It's a good way to get to that point where you can hear the whole thing and start to get comfortable with your view of it. Say loose while you're in the process. It's OK not to commit. Save variations as you go along in case you go down a wrong road or even just to compare and make sure you're still on the right one.
While you don't have this luxury when mixing live a lot of the time, in the studio you can take breaks. A lot of times something that was stumping you just needs a little coffee (but not too much) to jog it loose in your head. At the very least you can come back in and hear things a little fresher. Don't be fooled into thinking that working all night is what it takes either. A lot of times getting it to a pretty good point and then going to see your family or friends will let you relax and get it out of your system. Many times I've heard from people that coming in on the following morning after a good nights sleep they'll hear five things they need to fix and two hours later the mix is done to perfection.
Personally when I'm mixing a recording I do almost nothing until I have it at a point where I'm barely touching the faders. During that time I'll fix things that are obviously not right with some EQ and dynamics. Once it's sitting nicely then I'll go in and start placement with panning and spatial effects. Then the automation finally gets turned on. My moves at that point are able to be so subtle and smooth that it's more like the icing on the cake than heavy lifting to beat things into shape.