Watch the Dominant Substitutions online guitar lesson by Mimi Fox from Flying Solo
Dominant chords provide the most possibilities. Also the most possibility for error! If you are in the key of C minor, don't play a G13 chord because the "E" note clashes with the "Eb" of the C minor chord. Instead, use a G7#5 chord, or a G7b9 chord. Both of these chords add notes from the key of C minor (a D#,or Eb and an Ab) so this provides a smooth transistion. Remember not to get too busy and to tape what you are playing so you can begin to edit yourself. Just because you know 20 new subs to use doesn't mean you should play them all in one passage, or one song.