How to avoid overexposing sky in landscape photos

It depends on the situation ( lighting conditions ).
If the dynamic range ( difference from darkest point to brightest point ) isn't high enough, you can Expose to the Right ( EttR ) and drop the highlights and shadows to get your proper exposure.

If the dynamic range is too high, you take two shots, one exposed for the land and one for the sky and you put them together ( blend ) in Photoshop using Luminosity masks.

Hi Clive. There are many techniques used to balance your exposure. Firstly, shooting at the golden hour can reduce the range of stops of light in your frame. If your camera is capable to handling the dynamic range in the image, using the proper shutter speed to get the classic, bell-shaped histogram, is the optimal route. Since many scenes tests the limits of the camera, it is helpful to carry graduated neutral density filters to allow the use of a long enough shutter speeds to capture the shadows without blowing out the sky. Many photographer's prefer to blend exposures in post processing instead of using filters. This is achieved by capturing multiple exposures for each part of the scene. Hope this helps!!