Here are the actions I take to get the lowest possible action on my bass -- then if it's just not low enough I'll take it in to get a fret job if the frets are noticably uneven, causing fretbuzz.
http://www.garywillis.com/pages/bass...tupmanual.html
The basic ideas are:
* truss rod controls neck relief
* bridge saddles control string height off the body (bridge depth [lengthwise measurement] has to do with intonation -- bridge saddle height does not)
* nut slots being the right height (i've never owned a bass where this was done incorrectly, so can't make any recommendations on it)
I generally tighten the truss rod -- 1/2 turn per day to be safe!!! -- until the relief is about the thickness of a creditcard or thick business card. Some basses have such superior fretwork that relief allows the fret to all but touch the 12th fret. Find the relief by fretting the 1st fret, choking the last fret on the neck and seeing how high the strings are off the 12th fret. This is the first step, and should be done prior to any bridge saddle adjustment.
As soon as the relief is set, make your adjustment on the bridge saddles. Every bridge is different so I won't comment on HOW this is done, I hope you can figure it out
The basic idea is that you have to first setup the NECK which determines relief, and THEN the BRIDGE which determines the height between the body and the neck..
I hope this makes sense