Even the clock on the mantle was probably put there to help create the mood. Groupings make stronger presentations than single items. Group like metals, china and pictures. Keep the scale in harmony with the room and furniture sizes. The quantity of accessories used also depends on the mood and style.
Victorian settings thrive on clutter.
Contemporary settings call for a more streamlined look. Look at each area and ask what you are trying to achieve - a dash of color, an element of softness, something to fill a bare spot, something to add visual weight. Start with larger objects. Stand back and observe where the holes are and the n fill in with smaller objects. The use of color is not just a matter of personal preference.