Make sure your main subject is sharply focused. Try experimenting with different aperatures and use depth of field to your advantage. A shallow depth of field will have the background out of focus and provide good separation for your subject. A large depth of field where objects appear focused from the foreground off to infinity is usually appropriate for landscape images. Ask yourself if the scene would be better presented in a vertical format.
Vertical scenes depict strength, vigor and power.
Trees, tall buildings, people etc are good examples to turn the camera vertical. While horizontal is usually typical for peaceful scenic. This relates to filling your frame and isolating your subject. Too much empty space may detract from your photograph. Conversely, negative space can add mood or the sense of scale.