Here's how to build a stereo image that doesn't sound funny
when mixed to mono.  This is a great way to help separate
those guitars that are blending together too much.

It's a good technique to use in a lot of cases, and the resulting
stereo image can be panned anywhere in the mix.  It's one I used
quite a bit back in the good ole analog days and recently figured
out how to do in n-Track.  It works great with reverb, delays,
"doubled" tracks dragged a bit to cause a delay, and pitch-shift
doubling.

The technique is to put the dry signal in the middle, add the effect
to the left side, and subtract it from the right.  I've heard this
called "A+B/A-B", and it's also called "Mid-Side" because you do the
same thing you would to handle Mid-Side stereo mikes.
You can find the plugin at "http://www.mda-vst.com".

It works in any case where you have a mono dry signal you want in
or near the middle, plus a mono "wet" effected signal that you want
to use to create the image and add ambience.  I'll use reverb as an
example.

    In the track properties, turn on "expand mono track to stereo".
    Add any dynamics FX you want on the track.

    Add the reverb.
    If you want, adjust the FX to taste.  You can also do this later.
    Unlock left and right channels.
    On the LEFT channel, go 100% dry, 0% wet (that is, 0dB dry, -inf wet).
    On the RIGHT, go 0% dry, 100% wet.
    Now, re-link the left and right.  Make no further adjustments here to
       wet and dry levels.

Note that you want to use this as an imaging technique.  The effect you
use will cancel out in mono, so don't do it with an effect you want to
stay in mono.  That's why it's great with pitch-shift doubling.

To help separate tracks that blend too much, add different effects to
them, or use pitch-shift doubling with this technique.  (Pitch shift
doubling with M-S imaging is one of the best ways to take an old mono
recording or rhythm track and "pull" the different instruments apart
in the image.  Try it!)

For a fun trick, try changing the mode to "SM->LR".  This causes the
dry signal to get inverted on one side.  For a guitar with heavy reverb
or delay, this makes the guitar sound far away and the echoes close -- and
the further you get from the speakers, the more the guitar disappears,
leaving only the echoes.   Use this with caution, in mono, you only get the effect!

(I thought that Jeff Beck did this on Blow By Blow, in the intro to Cause
We've Ended As Lovers.  That's where I got the idea.  Subsequently, I found
that I'd had my speakers wired wrong at the time!)

You can also do this on an aux send, to add the same effect to a number
of tracks.  In this case, though, leave the dry part out entirely
(set M level to 0%).  Just as you would if you were only doing reverb
or delay or whatever on the aux send.

While few of us worry about our stuff being played on mono gear,
keep in mind that your music becomes mono when it goes through a
doorway, more or less.  If you're using phase techniques to create
an image, your tunes will get colored funny the further away from
the speakers you are.