Use
the contrast and brightness buttons to make as much detail from
the grayscale visible as possible, keep black really black, do not
let black become gray.
Note: In practice, the (two or three) darkest squares will look
completely black.
A simple way to adjust the monitor
1 - Turn
contrast all the way off (towards dark).
2 - Use
the brightness button to adjust the screen, so that black becomes
almost gray, but black may not show any light.
3 - Use
contrast again to make the white "white" again, keep an
eye on the black don't let it become gray.
Note: Don't make the
screen too light, there are several drawbacks when the screen is too
light, monitor flickering is more noticable and the screen appears
less sharp.
If you want more info about monitor
calibration, see Norman
Koren's great site.