Russ, here are a couple of 'on the fly' chord substitutions I often employ:

1) add a 'predominant' chord before a dominant 7th chord.
IIm - V7; IV - V7

2) Play a tri-tone (flat 5) chord substituton
Substitute Db7 for G7, F#7 for C7, etc, as these chord pairs share the same 3rd and 7th notes (a tri-tone interval) away notes.

There are many others as well and some of these are outlined here. Using chord substitutions can open the door to making a song more refreshing & uniquely yours. In addition to simply utilizing chord substitutions, it's the 'voicing' used (how you stack (invert) the notes of the chord) and the kind of (smooth) 'voice leading' played which can make or break a song. - Scott