Rather than have the horizon smack dab in the middle of a landscape style image try putting it in the upper or lower third of the frame. This perspective adds interest & impact.
Notice how all the heads are under the horizon
Their are exceptions to the rule such as in this sunset image where the reflection of the clouds in the sand requires the horizon to be in the center.
Notice the Horizon is equally offset by sky and reflection in sand.
See more portrait photographs here and landscape images here