Horizontal lines and horizons with a dash of diagonal.
Getting horizontal lines isn’t always easy. The first photo contest I entered one of the panel told me to use a level to get the horizon straight, since then I developed an abhorrence for crooked horizons, unless it intentional. It’s a simple fix that will make your photos better.
I did learn to use the level on my tripod and if I’m “freestyling” it I am at least aware so I work to make the horizon straight. If it’s not possible I leave extra room for cropping which is what happens when you straighten. If I do need to do post editing I usually use the straighten tool in Picasa (a free edit program) or the rotate canvas tool in Photoshop. With rotate canvas you can get a more precise adjustment.
click on each to see full photo.