Photoshop - Selective Masking for Adjustment Layers

Use this technique when you want to apply a correction to just part of an image
  1. Add the adjustment layer you wish to use but don't make adjustment, just click OK to remove the adjustment control panel.  This leaves the adjustment layer in place.
  2. Make sure the foreground colour is black and that the adjustment layer MASK is active (double line around the mask icon), and then press [Alt][BkSpc] to fill the mask icon with black.  No change is visible on the main image.
  3. Hold down [Shift][Alt] and click the MASK icon to reveal the mask on the main image as a semi-transparent red overlay.  *To adjust the intensity (and therefore the transparency) of this overlay see below
  4. Change the foreground colour to white (hit [X] on the keyboard), select a suitable brush, set the size and hardness and paint out the area(s) on the main image to which the adjustment is to apply.  Re-paint any mistakes with black.
  5. [Shift][Alt]-click the mask icon to remove the red overlay.
  6. Double-click the adjustment icon to restore the adjustment control panel and make adjustments as required.  Click [OK] when completed.  This action can be repeated later.
  7. You can restore and adjust the boundaries of the mask if you want to reduce or extend the area(s) affected by the adjustment.

* Set the red overlay to your preference: