For a single row panorama you can minimize this effect by having the horizon in the centre of the image and by eliminating close foreground objects.
If you mount the camera in portrait (vertical) orientation which you seem to be doing you are putting the long side of the sensor vertically to capture more height. This allows you to crop the top off, in post, to put your horizon back on the top third if desired.
If you do need close foreground objects then you will need a more advance setup. Google "nodal point" and expect a few hours reading.
Personally I have a simple setup with an L bracket and try to take shots without close foreground images. I have even had good luck just stitching hand held portrait orientation shots with a 25% overlap in Photoshop.IOM
, on Flickr
Lastly, maybe try a longer focal length to see if this will reduce the effect.