I'm aware of the benefits of using some gentle compression on the mix bus to provide a bit of "glue" to the sound and make everything work together a little better. I'm definitely up with that and can hear the effect it has very clearly. 
 
What I've always been confused about is this: what about music which has quiet parts and loud parts? In slapping a compressor across the master bus with a certain threshold level, the amount of compression during a loud part is obviously going to be a lot more than it is during a quiet part. But what if, during the quiet parts, you wish to apply the same "glue" (I kind of hate that word ) to the instruments? Is it commonplace to automate the threshold so that quiet parts get a little love too? 

Glue is pretty subtle, IIRC.  I'll ramble, albeit you may know more than me here.  Glue is somewhat unpredictable for me.
 
For myself I might just worry about loud parts being comp-glued, etc..  My ears might not detect where to envelope less loud parts appropriately.  Of course trial and error may work; the ears might be able to discern somewhat with cans, I don't know.  Such patient attention to details may be well worth it for your song ... especially if the song has long stretches of quiet parts.
 
(I've necessarily comp-enveloped single tracks for SALVAGING vocals at times.)
 
Other glues for less loud areas (not enveloped?) might be a 'tiny' bit of verb, exciter, widener, etc. with up to 10 millisecond delay or something ... for more consistent 'smearing'.  Ozone has these of course.  It also has "Intelligent" and "transparent" limiting which methinks also produces subtle glue (and coloration) proportionately (or something).
 
Some mastering limiters, iirc, use algorithms that detect lower dynamics and 'necessarily' put glue in the dynamics.  The Slate fx limiter, for example, has knobs for dynamics and transients to interplay.  
 

For me, the glue on a master bus is reverb.  That is, a room or small hall reverb set so quiet you can't consciously hear it but loud enough that your subconscious places all the instruments in the same "space".
 
I generally do this by having a "Master reverb" bus with a send on all tracks so I can vary the level of reverb for each track and vary the overall reverb effect with the bus fader.  You would normally have multiple reverbs across different tracks set much higher which will mask this "Master reverb"
If you need a track to jump out at you, automate it's send to reduce the Master reverb level.  If you've got the levels right you shouldn't hear a change in reverb (you shouldn't hear the reverb at all) but the track will detach from the rest of the mix.

Dynamics compression glue is still the preferred option for me.  Who says the buss compressor has to be set statically and left alone.  One thing you can do in mastering is automate things such as compression and even EQ for that matter.  No reason why you cannot do the same mixing into a buss compressor over your mix.  Just automate the parameters you need to for the quiet passages.  You can lower the threshold so the quiet passages also will just work the compressor (and output gain too) and before any louder bits come in you can raise the threshold back to your louder settings.  It is probably something best done using automation rather than riding parameters live although I have done that on a few occasions.
 
At least in the mix the automation will tie in with the grid. In mastering it wont usually but it can be used effectively too. 

Interesting ideas and concepts here. I guess I'd never really thought about using a master reverb before, or using exciters as glue. I've been demoing The Glue by Cytomic and have been very impressed with the effect it has on the master bus with about 2-3dB of gain reduction. Previous to that I was using Native Instruments' Solid Bus Comp which is quite decent, but certainly has a different character to The Glue. 
 
It's good to hear that some people are automating the compressor threshold though. I had thought "why not" but wondered if other people did it. 
 
 

You can automate the vol, too, to hit the comp harder in the soft spots.  I'm working on a spoken word project and have find out just how anal I can be.  Lots of time involved matching levels by ear, but it is a personal project so who cares?
 
@

I often use a reverb on a bus to put (most of) my instrument in a room, so to speak. When I say reverb, I really mean early reflections, mostly. In fact, sometime I simply use a delay. It's there to be felt, no heard.
 
It's something I've always done instinctively - it made sense to me. The principle was a good one but not the application -  I used way too much. My mixes have improved dramatically when I started using very short reverbs and started focusing on pre-delay/early reflections instead of the tail. 
 
As for mix buss compression, most of the time I pick a compressor based on what it does at unity, with no compression at all - like IK's Fairchild Model. At any rate, there's rarely more than 1-3 db of compression on the master buss, but even with no compression registering, bypassing the plug-in makes a world of difference.
 
I find that NI's Solid Mix Series works great for certain things, but it's nice to have a couple of options. I've heard nothing but good things about The Glue. I like IK's Buss Comp, the Neve model and McDSP's. Renaissance Compressor often ends up being my favorite - probably the most commonly used 3rd party here. Though I'm still using my bundled compressor most of the time. I'm sure I'd still be using Sonitus if I was working in Sonar. :P

I ALWAYS MIX INTO A MASTER BUSS COMPRESSOR.
 
it's just what i do.
 
 
certain songs, get a super gentle treatment...
and some get it more aggressive.
 
if i can hear it, i back it off.
 
but the 'effect' of it, is probably this 'glue' you speak of.....
 
 

I would agree that most of the time the reverb is very minimal, but I would also say that in certain genres and applications you want to feel like it's in a big space, so I think regardless of the amount used it can be a great way to glue the tracks together.
 
I always use a master bus compressor too Bat. I'm just curious though...are you using a separate compressor bus that goes to master? I have sometimes used both a separate master bus compressor and a compressor on the master. I might send a few tracks to it that I wanted the effect for. 
 
I find that the group function automation really shines for times when you might want to increase/decrease compression or reverb on a few channels that share a common trait. This makes a great way to glue certain parts in different places and make them  flow as one in the mix.

starise,
i just put a single band compressor on the master bus, and mix directly into it.
 
i usually have it set at about 1.5:1, no more than 2db reduction, and a fairly quick attack and release.
 
i have zero makeup gain, and the comp is either a Waves C1 or a RennComp.