Today I sat down listening to some Marcus Miller, the stuff from Sun don't lie album...
I noticed something odd in his songs when he slapped, sometimes he added a really soft heartbeat 'dum-dum'

I picked up my ibanez ATK and thought about how he got this nice sound.. after a while I figured something out. I layed my fretting hand softly on the strings to keep them quiet. Then I slapped the E string, like I would when doing a muted slap but instead of slamming it to achieve that "chunk" I softly striked it with my thumb just enough to get a sound.. not percussive at all, just kinda really soft nice sound.
This sounds kinda cool mixed in with regularly slapped notes and muted slaps and stuff...don't know if its the technique Marcus miller uses though...