It's like an ordinary screw (when looking on the truss rod head from above).
clock-wise: tightens truss rod - straightens the neck
counter-clock-wise: loosens truss rod - neck bow increases due to the string tension.
one 1/4-turn at a time.
Let it settle overnight (or 2 to be on the safe side) before turning again.
Gary Willis' website has some good info about bass setup:
www.garywillis.com