The "Minor Up A Minor Third" Trick
OK it's not really a "trick" but I've been using this a lot lately so I thought it was worth a note. The idea is that on a C minor you can pivot to Eb minor, and in general substituting "the minor up the minor third" sounds nice and unexpected.
I feel sure I've talked about this concept before here but I can't find it now so, briefly, here's the explanation. On a C minor tonality you can play C natural minor scale. If you move it one step round the Circle of Fourths it becomes F natural minor, AKA C Phrygian. If you go another step you get Bb natural minor, AKA C Locrian. If you go another step you get Eb natural minor, AKA... well it doesn't have a name, but that's what we're doing.
Why no name? Because it's a hypermode, not a "proper" mode. The scale of Eb natural minor doesn't contain the note C, which is our root note. But with a little courage we can just not care about this and play it anyway.
Currently I'm enjoying all kinds of chords out of Eb natural minor as substitutes or passing chords in a C minor context -- triads, sevenths, stacks of fourths, pentatonic things, they all work great. Eb harmonic minor works well too. Eb melodic minor is fine as well, but this is a mode -- it's C Locrian Natural 2, which you probably already know. Lots of other things work too, such as the Barry Harris sixth diminished thing. Just watch out the for major third (E) creeping in, which means avoiding anything in the Eb tonality with a b9, or at least treating it with care.
It works with single-note lines, too. Playing Eb minor lines and resolving to C, or to G or Eb (common to both) is very easy and creates surprising sounds. And it plays nicely with some of my other favourite things to do on minor chords, such as minor pentatonic with a flattened root.
Of course, "far out" ideas like these work when they're appropriate to the harmonic framework. Don't expect to do this kind of thing in a highly functional setting (such as on the ii of a ii-V-I) or where the harmony is moving quickly and needs a clear outline. But this kind of move is a great way to liven up a modal tune without the usual semitone side-slip, which in minor is full of extremely dissonant land mines for the unwary to blunder into.