Image straight out of the camera - note the sloping horizon

Using Edit/Transform/Skew to pull the left bottom down & right top up to straighten out the horizon
 Some might say that this distorts the image.  Alternatively, use Edit/Free Transform to level out the horizon followed by Crop or Edit/Transform/Scale to enlarge to eliminate blank spaces

Use Image/Image Size to set the image size to match the resolution of your monitor

Resized Image

Use Image/Adjustments/Shadow Highlights to adjust the these levels
This technique is useful in bringing out detail in a dark foreground (Shadows) and emphasising cloud detail (Highlights)

Image after Shadow Highlights have been applied

Use Background/Duplicate to create a second laver
Then set to Soft Light and adjust the Opacity to suit  - somewhere between 18% & 95%

Image after Background/Duplicate has been applied

Now Flatten the layers

Now apply Filter/Sharpen/Unsharp Mask to sharpen the image to suite
The values used in the example above work reasonably OK

Image after all processes have been applied including USM

Using the Spot Healing Brush Tool
Use the square bracket keys [ ] to make the tool diameter larger or smaller to fit over the spot, then click the mouse
Sometimes this doesn't work too well especially when the spot is adjacent to something else
Use the Clone Stamp Tool by selecting a section of sky adjacent to the spot, then press the 'ALT' key to 'grab' the selction, then move the mouse to the location of the spot and press the left mouse button moving the Clone Stamp Tool across the spot.

Using the Clone Stamp Tool
Sometimes it necessary to remove certain detail i.e. power lines that run across the sky or, as in this example, a garish image

Using the Clone Stamp Tool
Select areas of detail that match the background , then by slowly repeating the process, the character can be easily removed

Using the Clone Stamp Tool
Select areas of detail that match the background , then by slowly repeating the process, the character can be easily removed

Using the Magic Lasso Tool to tone down the background
Trace around the shape using the Magic Lasso Tool, then select 'Edit in Quick Mask' & use the Brush Tool to add/remove, then, with the Magic Mask Tool selected, use the Right Mouse button to select Inverse.  From the Filters Menu chose the desire filter (Gaussian Blur works well) & apply the amount required - not too much as it creates too false an effect, then the background can be darkened using Image/Adjustments/Levels
Alternatively, once the area has been selected with the Magic Lasso Tool, use Edit, Copy, Paste to overlay a copy on top of the original, then select the background layer to apply the changes leaving the copied layer untouched.

Using the Magic Lasso Tool
Here's the finished effect

Using the Lens Correction Tool
Most photographic lenses exhibit optical vignetting to some degree; the effect is strongest when the lens is used wide open & will disappear when the lens is stopped down by a few stops.  Optical vignetting causes a gradual darkening of the image towards the corners; this illumination falloff often goes unnoticed but it may become disturbing when the subject has large faces with an even colour or brightness.

From the Menu Bar, apply Filter/Distort/Lens Correction & adjust the Vignette/Amount sliders to suite
Chromatic Aberration (purple fringing) can also be compensated for using the Fix Red/Cyan & Fix Blue/Yellow sliders to suite

Using the Lens Correction Tool
Here's the finished result