Foreground, mid-ground, background
One of the simplest ways to improve your landscape photography, and one which also applies to many other types of photography, is to consider the picture in three parts; the foreground, the main subject area and the background. Having some detail of point of interest in all three areas leads the viewer's attention into the picture and holds it there as their eye scans backward and forward.
Having distinct a foreground and background also gives a sense of depth to your picture and helps to prevent your landscapes from appearing as simple flat snapshots of scenery. Using a vertical format can emphasise this effect, especially when used in conjunction with a short telephoto lens. This may seem like an odd combination for landscape photography, but the results can be very effective.
It's important not to overdo it though. If you have too much going on in the foreground it will detract from the main point of the image, and will be confusing for the viewer. Try to strike a balance between the different areas of your picture, to achieve a harmonious whole.