Turn monitor contrast all the way up as high as it goes. This is the control on the monitor itself, not the computer.

Adjust the brightness (on the monitor!) so you can see just a little difference between the black and the darkest grey.

That's it.

There are many more complicated ways of adjusting the monitor, but we have found this way the easiest. It is amazing how often you have to change the monitor calibration as light conditions change in your room. If it's bright sunshine then the monitor has to be turned up, at night you should turn it down.

This is a good way to know that your photoshop work will be in the ballpark. Link to this page and make it a favorite.