Find the Balance
Balancing the various elements of the shot comes to a photographer in one of two ways. Some photographers intuitively understand what makes a balanced composition. It is immediately and instinctively apparent to them as they choose which part of the landscape will fill the frame. However, if you asked them their secret they probably wouldn't know where to begin describing it.
For those fortunate enough it is an innate aesthetic ability. The other ninety percent of photographers, on the other hand, are fully aware of what balance is and does, understanding the techniques involved in balancing the elements of a photograph and are able to use these techniques to produce striking images.
Everything in the composition must have a harmonious relationship with each of its co-existing elements in terms of size, scale, color, texture, light etc.... If two objects or elements stand in stark contrast they will create a battle of opposition that will render the composition confusing. Of course you may want to have the focal point in absolute contrast to draw the eye but you do not want a number of elements in the photograph vying for attention.
Further, be aware of balancing the height of objects. If, for, example, the top of a tree in the foreground is precisely the same height as a building in the mid-ground or the peak of a mountain in the background then the image will flatten and the photograph will suffer.
Lighting Matters
Light is everything and in landscape photography it can be your greatest asset whilst also being your worst enemy. The intensity of the sun can, to a degree, be controlled by good use of the camera settings and intelligent filter choice. The sun's position and direction is something you can do nothing about except be aware of how this position affects what you see. Remember that the sun provides its best light during the Golden hours (an hour either side of sunrise and sunset).
To avoid images with high contrast try to wait for the sun to be over your shoulder because if the sun is too high or direct there will be a loss of subtle detail and you'll find issues with exposure.
Bearing in mind that different shapes, patterns and lines will work with you or against you to create a compelling composition that holds the eye, remember that as the sun moves, so the landscape will change throughout the day and the angle of light on objects will change too. So, if you have some outcrops or geographical features working in a diagonal path but everything else seems out of step then the likelihood is that at some point during the day the shadows will fall in such a way that they will compliment the shot geometrically.
Framing the Shot
Objects at the boundaries of the composition give the viewer a clear sense of what is 'in' your photograph. Don't allow objects around the frame be too indistinct or they will end up looking like accidental inclusions. Similarly, don't allow them too far in to the frame or they will unbalance the image by competing for prominence. Rather, aim for a compromise of the two.
Try not to allow framing objects to run the entire length or width of the composition or have these objects run in straight lines. Objects gently curving into the frame create a more natural proscenium for your work. Remember that the framing does not need to be a solid object but can also be a drift of focus. A rather harsh example of this is the use (and current vogue) of tilt-shift lenses.
When you are composing the shot think about how you might frame the shot in post-production with focus drift, tilt-shift effect or a gentle vignetting of color or contrast. Also keep in mind that you have the ability to crop the image later on. Leave yourself some breathing space and don't confine the image to the frame but rather allow the composition to sit comfortably within the frame.
Use the Right Lens
Although is is more usual to think of landscapes taken with a wide angle lens, it is always worth experimentation with a longer telephoto lens. This has a number of benefits but most notably a longer lens will tend to compress foreground and background objects closer together. The shot may not be particularly wide but shooting from a low angle across a long distance can create some interesting images. A good experiment is to use a longer focal length and create a triptych of three landscapes that work in both isolation and also together as a panorama.
Using a fish-eye lens can also affect the composition of a landscape by curving the horizon to give a sense of scale, and soften linear objects into smoother, more stylized curves. Although not optically very good you can buy a cheap wide-angle add on piece of glass for a few dollars which will thread directly on to your existing 18mm. It's not perfect but a good way to try out some super-wide compositions before you commit to more expensive prime glass.