Passing chords are pretty easy to get up and running by using simple 3 ideas.
First would be backcycling. This is picking a target chord, and by using the circle of 4ths, work your way to that chord. Use of diminished chords is also simple. As long as you have a whole step between chords, you can insert a diminished chord a half step above the target chord.
Finally, the use of chord scales is often overlooked as we tend to want more sophisticated ideas. However, chord scales are so inside, that coupled with other ideas, can make for very smooth movement. oleo20 already mentioned tritone subs, there are minor 3rd subs, and others, but really it doesn't have to be too complicated. Good luck. |