Click above or below the pointer to move it up or down by exactly 1 dB.
By turning the mouse wheel you can adjust the control in steps of 0.1 dB. As an alternative, you can use the cursor keys 
and 
. In both cases, however, you have to select the control first by clicking it.
By clicking on the numeric field below the control you can set it to 0 dB.
Some dialogs allow adjustments to be performed jointly for all controls while pressing and holding the Shift key. In the equalizer, for instance, you can set all controls to zero this way by clicking on 0 dB.
Many slider controls work in real time, i.e. you will hear the effect while you are setting the controls. Click Which slider controls have a real time effect on sound? to found out more.
Some of these explanations do not apply to the controls in the level display.