When you bring that camera up to your eye and line up that perfect shot in your frame… STOP! Take an extra look at the rest of your scene and make sure the background isn't going to ruin the photo. It's easy to pay close attention to the main subject, but everything else in that composition plays an important part in making a great photo. If the background is too distracting, it becomes the main subject — in a bad way. If the background is too empty, it can make the photo feel empty too.
I've ruined enough of my own photos in the past that it's become a habit of mine to double check my background and every edge of the frame. Our eyes are naturally drawn to the center of the scene, and it's quite easy to miss things along the edges or off in the distance. Don't let your photos get mucked up by unintentional background items — You're the photographer; you're in control.