I've been on a crash course learning how to create a good mix and I obviously have a long way to go. What I want is good separation, so I have been investigating how to accomplish this. I've learned a couple of things that have really helped with the latest song (I Thought I Knew) which I'll upload shortly.
I knew that basically I wanted to spatially separate the instruments to different auditory locations... the question was how the hell to do this. The biggest help I found on this was a LADSPA plugin from the CAPS Audio Plugin Suite called HRTF. This basically makes the audio for a specific track "appear to come from a specific direction in space". It works incredibly well for headphones, allowing you to specify a location for a specific track anywhere around the persons head. As the plugin readme mentions, it works only on a '0-elevation set' meaning everything will sound like it is on the same elevation as the listener - I'd like to find another plugin that also allows shifting of elevation. I've read a few negative comments about this style of spatial differentiation, specifically that since you are specifying a perfect listening location (headphones...) that it may compromise your mix when listening to it away from the perfect center. I haven't tested it in enough locations yet, but I know for headphones it works great.
Another hint I read was that you don't want to mix frequencies between instruments at specific locations. So, (my understanding is) you would not want two instruments with overlapping frequencies in a specific location or the mix will be lost. For example, putting the bass guitar 'near' the kick drum will not get the distinct clarity you want. Making sure these instruments have a well defined frequency space will help make a distinguishable mix. Unfortunately, in our case, the original drum tracks were all mixed down to a single track, so any chance a spatial differentiation is mostly lost, although some creative EQ'ing can regain some of it. Again, it sounds easy... the trick is actually doing this.
Another trick is called the Haas Effect. Basically, if you send the same signal to both ears - one with a slight delay, it creates the impression to the listener that the sound that arrives first is the only source location. Basically the brain will disregard/ignore the later arriving signal. This creates a very nice spatial effect. I used another LADSPA plugin called TAP Echo on the lead guitar and it sounds really great. Ironically, in much of our past mixing experience, we had discovered this effect by accident and used it extensively. We typically just duplicated the track manually or physcially re-recorded a separate copy of the track with slight differences /delays to produce this effect. With this plugin, it is easy to generate.
I should note that on my Fedora 8 box with the Livna repositories enabled, all of these plugins are available directly via Yum. Do a 'yum search ladspa' and you'll see what I mean.
Anyway, like I said, the latest mix uses these methods and I think has a much more distinct mix. Comments/suggestions/disagreements are helpful.