One of the key deciding factors about how good your landscape photographs turn out, is how you place the horizon in the frame. The decision is largely driven by rule-of-thirds, a well-known composition technique familiar to every serious photographer. Simply put, avoid placing the horizon right at the middle of the frame, which splits the frame into equal portions of sky and the earth. Instead, allow top one-third of the frame to be sky and bottom two-thirds be the ground or vice versa.
Whether the horizon should be at the top thirds or the bottom thirds can be decided by which part of the image — sky or earth — has more interesting details. If the sky is uniform and the earth has interesting subjects (hills, lakes, rocks,..), let the earth take two-thirds of the frame. If you have a dramatic sky (colourful sunset, interesting clouds,..), let the sky take up two-thirds.
Here is an example, where I decided to let the sky take up two-thirds of the frame. Photographed at Lalbagh Botanical Gardens, Bangalore.
Camera Data: Canon 5D MII, 17-40L at 19mm, f/16, 1/10sec, ISO 100, Singh Ray GND filters, tripod mounted.