Use the root and fifth of the chord at the current spot in your progression; i.e. in the examples I gave above, the outside notes, Aminor, would be A and E.
Sometimes you might you use the note a scale tone beneath the root - one or two keys below. So, for Aminor, that would be G
See example below i-bVII-iv-III:
[MP3]http://www.bandcoach.org/fp/audio/hiphop.mp3[/MP3]
---------- Post added 03-13-2011 at 05:57 PM ---------- Previous post was 03-12-2011 at 12:41 PM ----------
Thinking about this overnight, if you want to move from chord to chord with the bass leading the way, you might apply the same rule moving by a note that is a scale tone a way from the target chord.
So moving to the Dmin from the G you use a C in the bass line (a tone below the D) andmoving from the C to the Aminor you use a B (a tone above the A):
[MP3]http://www.bandcoach.org/fp/audio/hipHop2.mp3[/MP3]
Add 7ths and 9ths to your chords for more interesting sounds. Add some strings playing pizzicato and arco (plucked and bowed) and you begin to get a lush sound that can't be beat:
[MP3]http://www.bandcoach.org/fp/audio/hipHop3.mp3[/MP3]
That's Aminor9 (ACEGB) - G9 (GBDFA) - Dmin9 (DFACE) - CMaj9 (CEGBD)
Tie notes together that are the same to make the movement more smooth in strings if you wish or just voice it (vertically arrange the notes) as sounds best to you.