The most important thing to remember is to keep a common theme, or several common themes, like I did. A few uniting factors go a long way in making your pieces look like they belong together. My reoccurring aspects are- era (the 70's), color (blues, amber, b&w), materials (wood, brass), frames (silver) and theme (natural elements like feathers, bird, leaves, tree etc).
Most importantly- everything you use should be pieces you love and that have meaning to you. I recently purged some wall art that I had been hanging onto that I didn't love/had no real meaning to me. It made what I did keep seem all that more special. The three zodiac plaques are mine, my husbands and our baby daughter's astrological signs. They even say our signs are attracted to one another- pretty cute huh? As always, nature is a theme in our home, so the natural elements represent something loved and personal as well. I think the cool color scheme does a good job of balancing the fire of the red couch too.